I've been looking at this sword as well and as has already been pointed out, the horimono is naff, and glaringly so.
That, to me, doesn't prove a fake or a gimei straight away. It looks like a recent addition so that suggests it's covering up a flaw, although I'd rather look at the flaw than that horimono...
So with that said, what else about the blade suggests gimei?
Ignore the horimono.
Ignore his dodgy English; that maybe why he wrote 17th century Muromachi, doesn't really matter, who reads the blurb? (Apparently it's Nagahisa as well, not Hisanaga, so it would seem his Japanese isn't top notch either.)
Ignore the papers; I thought they were fake until it was pointed out to me they weren't NTHK or NBTHK.
I also have strong reservations about the seller from some of his earlier ebay introductions. I'd swear I've seen a couple of 'non Japanese nihonto' in his beginning auctions but I could be wrong, so ignore that too. :D
I'm curious as to the sword and what about it suggests gimei.
The nakago is well finished and the mei looks to be well struck.
The tameshimei appears to have been struck at another time from the mei, which I'd expect.
It looks to me like a very well made blade, although the polish/geometry around the yokote area seems somewhat rough.
I'm not looking to buy it, I'm just very curious as to its 'pedigree' aside from the horimono, and the fact that it is a cheap Kotetsu coming out of Japan.
Instinct says fake, I'm wondering what about the sword says the same?