The rules say that selling magical items (and most anything else for that matter) nets you half its market value. What happens though, when the players have used Ultimate Campaign's downtime rules to build their very own magic item shop?
Provided they set up in a large enough settlement, what is preventing them from selling their items at full value? How would that work out exactly?
Should players be allowed to do such a thing, and if not, why not? If you say no to your players, how do you explain it in-game? Why is it impossible for PCs to sell things at full value when the rest of the world has no trouble doing so?
Is this something the players do instead of adventuring?
Do they run this shop only during down time?
Have they flooded the market with the items they make?
How big is the market for what they are selling?
Do they have competition that can under price them?
Do they have to pay taxes?
Do they have to pay guild dues?
In Ultimate Campaign, it covers this in the magic item section. How much a person sells can be covered with the Profession or Craft skill as per normal. It represents how well they do at selling items and the fact that they have sold that item, but it's taking weeks for someone to be able to afford it. Having used it in a game, it's still a money maker, makes sense in game, but it doesn't get out of hand.
Is this something the players do instead of adventuring?
Do they run this shop only during down time?
Have they flooded the market with the items they make?
How big is the market for what they are selling?
Do they have competition that can under price them?
Do they have to pay taxes?
Do they have to pay guild dues?
My particular players have plans to operate out of Port Peril. Who is more adventurous than pirates?
In the case of a magic item shop, I would say sure, the PCs can sell their things at full value, but it's going to take a lot longer.
Most people are dirt-poor commoners, and can't afford magic items. Likewise most magic items in a store specializing in them are probably of little use to their owner (hence why he's selling them).
...Thus the owner of a magic item shop is technically rich, with all the magic items he has, but that richness means nothing until he can sell some of his wares. If the PCs open up a magic item store they're in the same boat, the value of their magic items means nothing while those items are sitting on shelves and waiting to be sold.
Owners of magic item shops are the ones willing and able to put up with this issue, I imagine. They make more by selling their items at full price, but it simply takes a heck of a lot longer to do it that way than to sell at half price like most adventurers do. They get their profits by buying items at half price from people who want money fast, and then hoping that something among those items sells before the rent money dries up.
If the PCs want to take that slow road then it might be reasonable to let them do so, but I imagine some magic items might take weeks, months, or even years to sell in such a manner.