It is very easy to solve that problem, the game can keep the soulbindness of the items, let's think of their intended use as helping the leveling player to get better gear, then if the intended reward is of lesser quality and the skin has already been unlocked, the character should receive extra money that would equate to selling that item to a vendor, not at the market, or an extra bag of loot, that way there is no fear that veteran players will keep creating new characters so they can run story line missions with the only goal to abuse the market. But I still don't get how can anyone try to use this path of enrichment that takes so much time to do that rather than just run around a zone and keep killing the champions that exist in the zone. Loot is so much easier to get that way, especially when doing lower level maps, when those champions can be single-manhandled, than to do long stories, listen to dialog and do all kinds of other activities, that aren't simple combat.