If Rib of Greed is picked up after a Greed fight in a Shop or a Secret Room, reentering the room will turn the room into a regular shop with items for sale or a secret room respectively (while keeping the previously dropped items on the floor).
By dropping the Rib of Greed before entering a shop, fighting Greed, then picking up the trinket again, the shop can be accessed while also having made free money from the Greed fight.