I wouldn't really require a skill roll except in extraordinary circumstances. The 4E economy is very precisely balanced. If the PCs end up being shorted significant amounts of cash because they suck at appraising/fencing loot, then that is basically the same as only getting 7 or 8 treasure parcels per level. That will eventually impact the expected power level, making standard encounters too difficult.
What I would do is use skill checks as a story hook. Perhaps a certain art object is "hot," and the PCs need to decide what to do with it -- return it to the rightful owner or fence it to the local guild or smuggle it out of the city to sell somewhere else? All three options open up story hooks for the campaign. You could also use gems that have some sort of possible side benefit to them ("Arcane lore suggests that lodestones have strange properties over magnetism and such forces"), possibly only discovered in limited situations (perhaps vs. certain foes or in certain terrain). There are plenty of possibilities. What I would not do is say, "Har har, you botched the thievery check and will only get paid 1/2 as much as is due!"