I don't much about 5e. I haven't played DnD in years, and the last version I touched was 3.5. So, I don't know if there's an appraise skill in the current version or not.
Regardless, it would be nice for this game. Anything to help with the terrible pricing at the traders. I get that you're not going to get full value selling the items, and I suppose I appreciate the marginal bump you get from characters with higher charismas, but if an appraise skill could add to that, then I think that would be wonderful.
All of that said, I don't get why the traders mark everything up so high. Sure, they pay you less than full value. I'm on board with that. But raising the price so high above value? That doesn't make as much sense to me. The value of an object is essentially what you pay for it in a store. If you pay, for instance, 1,000 gold for something then you can't turn around and stamp a 500 gold value on the item. Consider, you're selling it for roughly half "value," which means you're selling it for 250 because it's being valued at 500. Then the merchant is selling it for 1,000. That actually means you're selling it for a quarter of the value instead of a half. Even pawn shops give better deals.