Now, there would be some sort of trade value limit making sure an item is within the price range of that item. However, some exotics are better than others which means they would have to create a certain price range for each exotic. Sadly, getting this approved would be nigh impossible so it's never gonna happen but this is a snapshot of how it would happen. Then again... having an economy in destiny would be pretty cool, kinda mabye like an auction house or something.