Ah, the good old Who Sets Market Prices? question! I agree with Ron’s analysis for items that turn up regularly, but for top-quality rarities dealers can ask pretty much what they want and someone will pay it. That someone has been me now and again. I've never regretted paying over the odds for something I wanted and wouldn't see again. I have however regretted passing things up. I justify it by a) mentally offseting the wallet pain against the bargains I've bought, b) the enjoyment I get from owning a grail piece.
I've never bought from Weitze but I have bought from the UK equivalents. I've had some superb pieces from them, and just occasionally I had to return something. I believe that if you know your subject you can buy well from almost any dealer. A couple of years ago I bought a stunning badged 1st pattern Brodie from the seller of the “1842” above at quite a reasonable price. More recently, I had an outstanding bargain from one of the biggest rogues going (it might be best if I don’t name him here). I've had to pay the same tuition fees as everyone else over a period of years, and now and again I still get caught out.
If I wanted to make a living as a dealer, I’d be scrupulous about selling only genuine, good-quality unmessed pieces at top prices. Not many dealers seem to do that though. Too much temptation I suppose.