This is a take I've had on a post someone had made a few years back regarding magic shops, which is a topic currently at the RPGSite.
Wegmar is a tall, thin man, wearing a silk smoking jacket and a top hat. He is loud, waves his hands a lot, and will try to sell a glass of water to a man standing next to a river. He has a large, enclosed wagon drawn by two small gray mules. He travels the dimensions, selling odds and ends and magic items.
The first sign he is near is a glowing blue magical circle appears on a nearby road. Then the sound of wagon wheels rumbling, and his wagon appears. The gate vanishes shut behind him.
However, he does have a small selection of magical components, a few potions, and a few scrolls. He's also very likely to have some kind of miscellaneous magic item, such as a crystal ball or rope of climbing. He does not sell weapons and armor, magical or not.
Wegmar accepts coin for non-magical doodads, since they don't really cost him anything to acquire.If a customer wants to be something that is actually magical, he prefers to either trade for another magic item or he will accept a favor, to be collected at some later date.
The favor will be collected at some later date, and is generally going to have the players going on a short adventure to recover some magic item that Wegmar wants. If the players try to attack Wegmar, he and his wagon will disappear, never to be seen again.
Any players who owe him a favor must fulfill it when he asks, or they will suffer a non-removable geas that gives them a -1 to hit, damage, and all saving throws until they start trying to complete the adventure for Wegmar. After they acquire the item he wants, his wagon will reappear near the adventure site and he will collect it.