A subset of traders here like to mark things shipped as soon as the trade configuration is finalized, to minimize the risk of trading the same card twice (inbound or outbound). The ones I've interacted with have always done it only with mutual consent, however.
I wish there was a button to remove from tradelist and inventory, instead of linking it to the action of clicking "shipped". I go through and do it by hand, but it is a real PITA on a bulk trade of a whole pile of cards!