Letting your party enjoy their wealth & buying magic items

[Unoriginal]

In Tomb off Annihilation, the mage Wakanga O'tamu has one shop where he sells scrolls up to second levels and some potions, and he also knows the magic item formulas for +1 blades and +1 shields, for which he's commissioned.

He's one of the Merchant Princes of the ploutocratic Port Nyanzaru, he's richer than the temple of the goddess of money, and he has a monopoly on the magic item trade in the area. Him and his colleague Merchant Princes are rich enough to keep a Sorcerer with 7th or 8th level spells on their payroll.

In Forge of Furry, a smith is utterly legendary for the weapons they produced in the eponymous forge before they were killed, and one of the quest givers is willing to spend thousands on even one of said weapons, yet none of them has powers stronger than a +2 bonus.

Yes, buying magic item is possible, but anyone capable of selling them regularly is rolling in dough one step removed from the adult dragon hoard, if not above.

Though of course it depends of a lot of factors. A Cape of Billowing is awesome, but it's Common, it won't fetch for a ridiculous price unless there is some story or the like making it more important, like a famous crafter or previous owner. But more than anything you have to be careful to make sure you're not buying something stolen from someone who has the means to get it back, and that the item isn't cursed. I tjink the rules in the Xanathar's work for that.