It is your obligation to ship the item to the buyer who has already made a payment for your item. If you have been in negotiations with another user, we would advise telling them that you have prior selling obligations.
SidelineSwap is a buying and selling platform first and a trading platform second. When a buyer has purchased your item at its "Buy Now" price, that item becomes their's.
SidelineSwap reserves the right to remove accounts of those sellers who refuse to ship to the buyer.