That dealer does 'sale or return', so I wouldn't be surprised if they are selling on someone's behalf. It would ring a few alarm bells if it were me, from what you are saying about it.
I'll probably get shot down for this comment, but at nearly 90,000 miles, it is getting on a bit, and you might end up with the same problems that are bugging you about yours. Having said that, there aren't many low milers up for sale, so it's a difficult one, but without any paperwork???
I would keep looking, rather than getting drawn in any more, but of course it's your money...