Posted By:
NickI would add these other situations:
If I find something that I consider very valuable and no price has already been marked with it, I ask the seller how much; I don't make an initial offer.
If the seller doesn't come up with a price but asks me what I'll pay for it, I make a pre-eBay style offer (i.e., what a dealer would have paid to a knowledgeable private collector selling cards back then), which usually amounts to about 1/4 to 1/3 of the item's normal selling price.
If the seller comes up with a price (either pre-marked or when I ask), I don't normally feel any qualms about buying from them if they set an extremely low price that indicates they have no idea what they have. There are exceptions to this. I have on a few occasions told little old ladies manning the checkout table at charitable rummage sales that something was worth far more than what they were asking.
If it's an estate auction, I treat it just as if something is put up on eBay for $.99 with no reserve - if I get a great buy, it's the seller's bad luck.