Look at his feeback. I'm not sure if this is an issue but he has sold two transmissions this month. Thats it, then the last time he sold something was Back in March and you can't even see what the item was.
I wouldn't say thats really a bad thing though considering what the product is. Probably not a lot of people buying transmissions off of ebay all that much. The guy looks pretty legitimate and it seems that all of his customers were happy with the product.
Also, since it may not bee easy to tell how well a transmission is doing after only having it a few weeks I would suggest contacting some previous buyers (if you can, I don't know if you can do that on ebay, but I would assume you could) and see how the transmission is working out for them.
Plus the guy is offering a two year warranty, but be careful with that. Make sure you find out exactly what the warranty covers becsaue it may not be as good as it appears. Well I guess is does say Replacement warranty, thats pretty impressive, I'd say that it's a pretty good deal.