One good marker here come from this question. That question is point to Wizard's official Adventurer's League material, part of their Organized Play.
At the end of an episode or adventure (and sometimes during an
adventure), you can buy or sell mundane items. Any gems, jewelry, art
objects, and trade goods you find during adventures are automatically
converted to their full value in gold and divided up among the
adventurers in your group. Arms, armor, and other equipment fetch half
their cost if you sell them.
There are some key elements here. Things found as "loot" are meant to fetch full value. So if the PCs find 5 diamonds worth 50gp each, it's expected that they will get 250gp, just as if they had found 250 gold coins. By default, the "art" and gems stuff is mostly there to provide flavour rather than forcing PCs to haggle at everything. Likewise the weapons are not intended to be a revenue source, so we just assume that they are damaged and don't carry full value.
Likewise, D&D 5e has a very stable set of basic living expenses for PCs based on their lifestyle. The lifestyles in 5e have a specific cost and a good idea of what that entails. This means that you don't generally need to do things like haggle over the price of a room at the inn. If they go to the right inn, we assume they're getting the appropriately valued food, drink & lodging.
All in all, I typically take the stance that things that are "used" sell for 25% to 50% of their original value. This enables the merchants to have a markup when they eventually re-sell the item. It also discourages excessive "trash collecting" by players (unless it's part of their character). If someone specifically has a merchant or trader background, then maybe there's some leeway of 10% either way on the pricing.
I think if you set something like this as a default, then it enables you to handle pricing more specifically on things that really matter. Trading magic items, negotiating prices for that guide to the forbidden lands, buying a keep, splitting up earnings from the mine you just cleared... I think these things can be fun for players as long as they're important. Otherwise you just follow the defaults and it's easy for everyone to operate.