I find that the extra bookkeeping of keeping track of all this sort of valuables and how to convert it into cash is more trouble then it is worth. It also tends to slow down the game dramatically and players ask about if/how literally everything in their environment can be converted to cash.
When I want to add flavor to treasure of an encounter would describe it (a beautiful painting of the Count's grandmother, rumored to have been part fey) but I'd just have the players record it a $500 gp (or whatever) and move on. No bothering with trying to appraise it, or searching for a buyer or anything like that. Insert the flavor when they find it, but after that it is effectively just coins.
Obviously this is just a matter of taste and there isn't necessarily a 'right' way, but I have seen you complain about your party moving 'slowly' before, and I can't imagine that having to figure out if a Minotaur's spleen has any value and where the best market for it might be doesn't contribute to a slower paced game.
I partially agree. Routine items they know the value of. Artwork or the like they know the value of when they get to town (unless they have the skill to figure it out in the field, which I have yet to have happen.) Converting it to money is automatic in town. However, in the meantime it has encumbrance. When it's trade goods or the like they very well might be faced with more loot than they have the ability to haul off.