This is like selling back furniture or customs. There would be an option to sell items or hats you no longer want. It should ask you to confirm that you want to sell it and tell you how much you're getting back for it. An extra notice should appear if the item/hat is limited time only (from Nexus/Revolution/Club Crypt/Onnet General Store/event, etc...). If the item/hat is still in that store, the notice will say "This item is limited time only. If you sell it, you have [insert number of days item will still be in store] days to buy it if you later decide you want it. Are you sure you want to sell it?" If the item/hat is already done being sold, a warning will appear saying "This item is no longer being sold. If you choose to sell it, there is no way to get it back. Are you sure you want to sell it?" If it's an event only hat, the message will say "This hat is event exclusive. If you're lucky, you may win it back, but it's unlikely. Are you sure you want to sell it?" Items you can't sell this way include bombs, arrows, VIP items, trade quest items, bugs, and hats that every player gets when they buy the customization pack.  

One's that you pay for, you get money back.
Ones that you don't pay for, you don't get back.  

Hats would turn into furniture. Then people would sell them all so they could get some gralats  

I would be a fan of this....