1)    It is forbidden to buy even something which has been put up for sale and looks like it has been stolen (for example if the seller takes it from a hiding place and cautions the buyer to keep it in a hidden place etc.). (Kitzur Shulchan Aruch 182)

2)    However, if the buyer buys it from a thief only in order to return it to its rightful owner (if the owners will pay him the amount it cost him to buy it from the thief) it is permitted. (ibid)