eBay makes it incredibly easy to check past sales figures, as well as to see many times unsold pieces have been relisted. Other auctions also make this information public.

So is why are so many eBay dealers insisting that their identical (or sometimes inferior) pieces are worth 2-4x more than they actually are?

This is the second time today where I've gotten into an offer/counter-offer situation with someone who doesn't understand that they don't control the market value.

Common sense note: the existence of a bunch of high priced listings doesn't mean that your piece is worth that much--it means that people aren't willing to pay that much, or else they already would have.