Yeah, it's mine. Though the price should be half of the selling price that you are selling (We should have a shop for items). When a user buys from a shop for an example, he pays it 1k. He can sell it for 500. if another user buys from them that rule doesn't affect them.

Maybe power can be used for buying special items.
For an example, I have 20 items and about 40 power. I can give my power for a special item. The items I had go back to the shop.

Maybe you can return your item for 500 but you should be allowed to sell it to another users for whatever you want.