They wanted a lot of expensive crap for the player to spend on, so they created random enchanted stuff with random names, with no real thought to it. Its how you end up with an "akaviri warhammer" that is clearly Dwemer in origin. There was no reason to explain how they get this stuff.
For an in-game explanation, most shops tend to carry a centerpiece. A massively expensive curio or antique that will only be moved by the ridiculously rich. Its a way to attract attention and draw people to examine their shop.
As for the warhammer, you don't know the thing's origins. It could've been pillaged during the Potentacy, enchanted and used, then been part of the spoils of war from the invasion of Ionith.