Christoph Laureys :- Seller is advertising this as a shinto wakizashi made by Tadatsuna (unpapered) I have no material at hand to compare the mei with the real deal though.
Can anyone shed some light on this question?

Darcy Brockbank :- I will declare this sword as certainly gimei and I am going to use an unusual method as my rationale. Note the location of the mekugiana in the sample, then note the placement in Ludolf's chart.
The smith in the sho-shin examples has chosen the intersection of the upper left corner of the ta character 田 as the center to place the mekugiana. Or we don't see it at all... I will get to those we don't see in a moment.
In my Bizen book there is an example of a Sukemitsu where the smith has placed a single tagane to mark where to drill the mekugiana, but the person placing it put it somewhere else (probably upsetting the smith!) We take special note of this sword, because it tells us something we would otherwise not know: the smith took care to dictate where he wanted the mekugiana to go. Someone, not the smith (otherwise the instruction would not be needed), was expected to place the mekugiana in the correct place.
It shows though that the smiths do plan for mekugiana, and the pattern in these signatures to me is just too strong.
The picture that Nobody linked also shows the mekugiana placed the same way, at the corner of the ta.
Flipping open Fujishiro, page 128 and 129 shows three more examples where the mekugiana is placed identically at the upper left of the ta, nicely centered using the upper left corner as the placement center.
Three further examples are shown where the mekugiana is nearly placed to the upper left of the first character in the mei. I am willing to bet that Ludolf's sho-shin examples that do not show the mekugiana, will all have the mekugiana in this same location at the top left just above the first character in the mei.
My conclusion is that this smith and his son (Shodai and Nidai) strictly followed rules for the placement of the mekugiana. It is either to be intersected on the upper left of the ta, or to be placed at the upper left above the first character.
The only sword that does not meet the pattern is the one in the first post. This mekugiana is placed carelessly compared to the rest. The person making the mei from a good example would have taken care to match yasurime and signature strokes as best as possible, but would not realize that what seems to be a random placement of mekugiana from one example "carelessly" intersecting the mei is actually part of a strict formula. I didn't even think of it until now, until looking at Ludolf's chart.

I say the sword at the root is gimei, and I feel that I don't need to go any further in my argument other than the placement of the mekugiana. After making my statement, a quick sweep through google:

Upper left placement:
http://www.sanmei.com/Pictures/Sword/N12962_W6940.jpg
http://www.sword-art.com/www1/wraiten/speker9.files/9-2.jpg
http://sinogi.ddo.jp/syasin/osigata/katana/tadatuna.jpg
http://www.e-sword.jp/sale/0650/jpg/2005main.jpg

Corner "ta" placement:
http://www.ricecracker.com/japanese_swords/s2/s2-tadatsuna/s2d_tadatsuna.jpg
http://www.japanesesword.com/Images/Swords/04/604/604wak1/DSCN3475.jpg"
http://www.adcoop.co.jp/j-sword/j-sword_hi/148_hi.jpg

No examples are shown that are not one or the other.
QED, first sword is gimei. 

Koichi Moriyama :- I see a clear difference in writing of the lower part of 近. 

Nigel :- I agree with that, Moriyama san... now I am studying these examples more I do see differences....
But if I look for differences between some of the shoshin mei examples I start to see irregularities there too...esp the 2nd to last character....

Darcy Brockbank :- The variations you're noting is because the smith is making them at different ages... as he gets older, his hand strength will first quickly become stronger, then will taper off. His skill with a chisel will continue to improve as his hand strength decreases, then this too will taper off. Finally you have the difficulty of not being a robot.
So some characters will vary, and the skill is in trying to find what just is wrong because a forger could not emulate it correctly, vs. what is the natural variation. A lot of this is an art. There is a nice page that Fujishiro compiled of signatures of Nosada that have I think about 50 nijimei on one page. There is a LOT of variation within that large of a sample.
What tends not to vary are the objective manner of finishing nakago. There may be a stylistic sea change, for instance in the nakagojiri of Omi Daijo Tadahiro. But once this is finalized he stays within the style. Yasurime tend to stay very true to school or teacher.
Another thing to check is the relationships of characters to each other. I took examples from Fujishiro with this signature style, and looked at the last three characters, and then the questionable mei. In particular I noted that the characters were lined up nicely with each other in the sho-shin, in the questionable mei they started lined up on the shinogi and the last character then drifts off. So they are not following this line. I dropped vertical alignment references in the middle of the Tada character, and noted the difference in the placement of the Kami above and the Tsuna below. In the questionable one, the Kami is drifted to the left, and the Tsuna is drifted to the left. Even within the Kami character itself, the relationship of the main vertical stroke is drifted with respect to its atari which falls on the line like the others.
I left some cyan markers to compare certain atari between the characters. Atari are usually where the largest mistakes will appear. In this case there are shape mistakes, direction mistakes, and there are gap mistakes. 

Now about my first mekugiana posting, I have pulled out Ludolf's examples and Fujishiro's examples and the questionable mei. I have placed a red crosshair over the mekugiana. I placed a cyan dot at the approximate crossing over the horizontal and vertical strokes of the mei. In some cases the lines are more curved then others which may alter the placement. In general, the approximate location of the intersection is very close to the crosshair center, with one exception, the questionable mei, where the intersection actually lies on the border of the mekugiana. I certainly think that there is a clear difference in layout between all the sho-shin examples, and the questionable mei.
Hopefully you will find the clear illustration more convincing

Here are more things you can look for. The worst atari error in the questionable mei is in the Tada character. Note the atari that is indicated, and where it is pointing. In each sho-shin example, the sharp end points directly at the atari above it, that terminates the bottom stroke of the Tada.

In the questionable example, it is pointing and attached in the middle of the curve of the stroke. Missing atari, atari located in the wrong place, oriented poorly, these are all signals and important ones in looking for gimei. Just one is not a big deal, but overall you're looking for a pattern, and in combination with other problems.
Like the example above, a major one is looking for smiths who line up their mei with the shinogi. Tadatsuna is lining up his characters nicely with the shinogi. When in an example the characters are drifting off the center line of the shinogi, or are drifting left and right in relation to each other, it is an example of someone who is working off of a reference mei. Someone carving their own is concentrating on putting things in the right place. Their reflexes make the characters correct. Someone forging is concentrating on emulating individual strokes. In the complexity, what you get is strange spatial relationships, things drifting off center and so on.
Probably the most important thing to look for is confidence. Confidence is seen in varying strength of the strokes. A confident mei will flow and the strokes are going to be of equal strength. Lines begin and end strong. A gimei will tend to have varying strength in lines as the smith goes back and forth with his template and is unsure about what strength at any one point is required to perfectly emulate the stroke. He is concentrating on emulation, the smith is using reflex. So the emulator is going to vary, too soft, too hard, too soft, too hard. Sometimes badly.
Consider the following: 

In the context of this one character, the smith has shown four different line strengths, from very weak, to very strong. When comparing to the sho-shin, the characters carry the same strength throughout. The "VW" above is particularly bad as it narrows in the middle then widens as it continues.
Go back now and look at the atari in the upper left of the Tada. No confidence. The more you examine this mei in detail, the more it falls apart. This of course is never the goal of the forger, he is not trying to stand up to close scrutiny because he knows he can't do it. The goal is plausible deniability. Good enough to pass, or good enough to TEMPT. And that is the real killer with well done gimei, is that the temptation on the part of the buyer makes him want to believe in what he is seeing.
One has to step back and look at the sum of the parts in coming to a conclusion on these things. When you have a couple of strikes, no big deal. But when the mekugiana is wrong, the characters are misaligned, they drift off the shinogi, the atari are oriented incorrectly or (worse) missing, the characters are not all completely confident, it paints a picture.
I highly recommend anyone interested in studying mei to get the Nihonto Koza volume that discusses this, they cover all of these subjects. I believe it is the Shinshinto volume, and you can get it at www.afuresearch.com. I am no expert in these matters, and I am way out of practice, so this was good exercise.
I stand on my mekugi ana argument without all this additional fluff though