Some creatures that can be summoned, like Ice Devil, Erynies, etc, has magical or masterwork equipment. Can a character summon one of those (via summon monster spell), ask him his equipments, dismiss him and then sell the equipment?
No. It all disappears when the summoned creature does.
However, you could theoretically call (via the planar binding spells) the creature in question then take and sell its stuff. If you do this regularly you'll either need to kill the creatures or when you dismiss them they'll be pissed. Do it enough and you've got a lot of angry outsiders to deal with.
Edit: also the DM is liable to screw with you if you try this as a money making scheme.