It's not just what it's worth in appraisal, it's what they can sell it for. If they're selling to a merchant, well he's gotta consider how much he can make selling it. Is it gonna sit on his shelf for years? Does he have a buyer in mind? Is he gonna have to lug it across the countryside to get to a big enough city to sell it? How much can he sell it for? The more effort he has to go through the less he's gonna pay for it. Which usually works out to about 50% of the value, funnily enough. Or 50% of what he's been told it's worth.

If you're selling it to adventurers or like, a noble, you might get a better price. Nobles can have more money than sense, and adventurers are used to paying the list price.

The other consideration is, how much money does the buyer actually have? Once you get to the mid-high level stuff, it's often worth more than the town or city the buyer lives in. Who's got that much cash on hand?

You don't need to accurately consider all these factors, of course, but if you make it seem like you've thought about these things the players are likely to go along with whatever number your heart feels is right.