as soon as you get an axe in the bazaar that does something different than other axes in the game, your decision to use the word skin will be moot, because anything and everything you say will be drowned out by an ocean of complaints that the new axe is pay 2 win.
bazaar stuff is all cosmetic. by your definition, the building sets included in the DLC are skins of the vanilla T3 building set. call them skins, call them cosmetics, call them whatever you want. but please dont ask for bazaar-exclusive functionality of items. please.