This is a general statement, understanding that there may certainly be specific examples that are exceptions:
Study as much as you can, gather as much info as you can. When it comes down to it, prices on any secondary market are speculative, that is, not fixed by a retailer. You need to educate yourself, so you have an idea of what the market will bear, before you bid in an auction, or haggle with a dealer at a show or flea market.
eBay is a ready source, but it is by no means the exclusive source. If you can, check auction catalogs, online or hardcopy, and follow up after the auction closes. You'll develop a feel for what the market will bear.
If you're buying from a dealer and not in an auction, remember that the dealer has to buy low and sell high, to make the profit that enables him to stay in business. But also remember that you'll probably be able to negotiate or haggle, too.
In the secondary market, it all comes down to what the seller and buyer agree to as their price, at any given time.
Prost!
Brad