Are you obligated to return an item purchased at a used goods store if someone donated it by mistake?

Say you saw a striking antique vase at a thrift store that was marked $50 (and well worth $200). You buy it, then a few weeks later go back to the store. The proprietor sees you and asks you to return the vase because the person that donated it said there had been a "mistake" and wanted it back because it was a valuable family heirloom. Would you agree to return it if either the shop owner or the donor promised to pay you back the same amount you paid? Would you ask more than you paid? Who is more responsible for the reimbursement? Would you give it back even if either won't agree to reimburse you?