I find this sort of discussion interesting. I'm relatively new to nihonto, and have been finding swords for martial arts, but I've bought from four or five different sources over the past year that mostly sell art swords. I have absolutely noticed some particulars when it comes to what one dealer might mention versus another, and have had a few unmentioned flaws on the swords I've bought.
For example, one had a kitae-ware along the mune, but it was so well hidden that I didn't catch it for weeks, since it's iori mune, and the flaw was on the raised portion. The same had a little pitting in the shinogi-ji. Neither of these things was mentioned, and I couldn't spot them in the photos, even after receiving the sword, but I'd classify both flaws as minor, and didn't feel the need to say anything to the dealer.
Another, a wakizashi from one those e-auction dealers, came smelling strongly of rust, with a big glob of something (maybe gummy adhesive stuff) near the kissaki, along with some fresh, shallow rust in the monouchi area. Again the photos showed none of the rust, and the koshirae looked cleaner/less beat up in the photos provided.
My latest purchase still has some very faint, only-visible-at-a-certain-angle cutting scratches in the monouchi area, but it was clearly marked for iai/tameshigiri, so I wouldn't consider this a flaw. Just something unmentioned and not picked up in the photos provided.
With all of that said, my experience in the sub-400k market has taught me what to expect for "junk" swords. Almost everything I've bought so far has been direct from Japanese dealers, all online. In all cases where koshirae was included (all but like two), at best, it was crappy but usable (for martial arts), and at worst basically falling apart. All of it is paneled samegawa, all but one were plain black saya, possibly with nicks and chunks missing from bad resheathing, definitely with some scratches and dents.
The tosugu have been a mixed bag as well. On one, I looked at them and quickly found a site that sold the exact models. Another's fuchi and kashira felt like plastic (very, very thin). The ito is mostly fine, with all but one being silk. The mekugi situation has been a little...strange, though. One had a tapered metal (soft with a lot of dents) pin, with the hole entirely too large on the other side. One had some sort of black...plastic or something put in place of bamboo, and the others range from totally acceptable mekugi to "that's going to break if I don't replace it."
Ah, and one still had a price tag (I'm assuming from a sword show) on its bag for like 80k JPY, when it was sold for like 260-280k. To be fair, I don't know what shape it was in before the dealer got it. They very well could have had it polished, since it was exceptionally sharp, and in a very new-looking polish. There's also a chance that the bag was from a different sword, so I can't say it bugs me.
At any rate, someone far pickier or caught up on the principle of the matter might deem any number of these things unacceptable, but for my criteria, and the fact that I'm seeking out swords collectors wouldn't want, they're fine. I will also add that, by getting experience with (relatively) cheap nihonto, I am able to learn from them in a way that I believe would give me a good head start if I did decide to start collecting art swords.