If a seller leaves a positive feedback it does not mean a deal is done. They still have the normal time period (45 days i believe) to file a claim for whatever reason.
I once had an individual purchase an item from me on ebay and leave a feedback when he received the item (within a few days). 25-28 days later i went on ebay to find this person filed a claim for a damaged card. Long story short i lost the case and refunded the buyer.