Unlike other commodities, a vendor NPC cannot add an owner's mark to a cart sold. This works with other commodities (armor, for example), but not with carts (maybe because of the "ghost cart" in the vendor's inventory).
The solution is to use a third party (an invisible mob for example) to apply the mark. mposet cart mark $n should target the most recently added cart and not others already in the room.
Owner's Marks on Carts
Owner's Marks on Carts
"A man may die yet still endure if his work enters the greater work, for time is carried upon a current of forgotten deeds, and events of great moment are but the culmination of a single carefully placed thought." - Chime of Eons