It's just similar to how traders operate in the real world. Ideally, merchants will purchase goods that they can sell as fast as possible for a profit. If they can't sell these goods fast, their profits will drop - a large amount of money will be "bound" into them (and thus can't be used to make even more profit for the duration in which they are stuck with it), and on top they have to spend a lot of money for keeping them secure. Thus, if they are unlikely to be able to sell the items soon, they will only buy them at significantly reduced prices.
And purchases from adventurers will often be seen as suspect. "I found it in an abandoned ruin" sounds rather similar to "It fell of a truck" or "I inherited it from my dear old grandmother." Would you trust adventurers when they claim that there is no catch to the items they sell?
No, with items purchased from adventurers there is always some sort of risk that there is more to the item than it seems - an enraged former owner, which might be anything from another merchant the party rogue stole the item from, to a not-dead-enough lich whose tomb the adventurers disturbed.
Thus, magic item merchants will use some sort of risk assessment when purchasing items of unknown quality - just like real world traders, banks, and insurance companies do. They need to balance the possible risks inherent in purchasing the item with the possible profit - and that ultimately means that they need to increase their profits. Thus, they will offer less money for the items.
Hmmm... Maybe I should write up some sort of rule system for magic item trade in Urbis to take all these things into account...