We took an idea from somewhere else: The Delver's Guild.
The Delver's Guild is a loose association of fortune hunters, explorers of old ruins and general ne'er-do-wells. In short, Adventurers.
The Guild will help someone find a buyer for that minor or inappropriate item, and help PCs find items they need. For a small percentage, of course.
They're likely to have a number of low level scrolls, potions or oils of the type seldom used, that various adventurers have traded in for a fresh stake. Popular ones like
Cure Light? They can get those for you, at a small markup. Might take a day or two.
The guild hall has a bar, of course, and a common room, but few commoner's here. Depending on the size of the city they might have a few rooms to let, and their stables can cater to exotic mounts.
Oh, and it's a great place for plot hooks.
As for the items themselves, per the book the default price is half purchase, as a base. Diplomacy can run it up or down, of course, but try to Intimidate the Bursar and you can find yourself banned.
Note that they will seldom pay you the best price in town, and they'll seldom have the lowest prices for any kind of magic, but they're more likely to have the odd dose of Stone Salve, or the Quall's Feather Token of an oak tree that nobody else seems have even heard of.