There is a fee for using the NFTmart but it is usually for the sellers. A portion goes to back to the game and the other has a portion that is sent to the marketplace makers. Then again, I haven't checked it in a while but I believe that is what happens.
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You are totally right, @jfang003! I will update my post.
There was a fee deducted from what the seller gets. Same fee structure as for the native market: a part going back to the game, and a smaller fee for the agent (this time the nftmart.fees)