I think some good points have been brought up here: there's a lot of junk minis on eBay. Well, not necessarily junk, but "lower quality minis". Some people ARE looking for those, for gaming purposes for example. The problem is that lots of this crap is titled "pro-painted". A user looking for pro minis would never buy such material. It's the same as including popular keywords, such as mp3 and sex, in your (for example mini related) site to get more hits through search engines. Someone looking for an mp3-site and who comes out on your mini site will close that window faster than he opened it, plus it's no good for your reputation.
That's probably why popular sellers keep getting reasonable sales, while other people, who paint equal quality, get less or no bids. I think buyers have a couple of favorite sellers they monitor, to avoid the other crappy painted stuff.
But it could also be that the market is getting somewhat saturated...