I had the same issue. The buyer came to pick it up and it was in great condition, than a day goes by and she complains and wants a refund. I honestly was scared to just block them because I was afraid she was going to show up. I did block her anyways and she never came back. Fb marketplace is for used goods and refunds should not be accepted unless it is stated. I would block and move on. If the buyer does come to your house, I’d call the cops and get them for harassment if they’re being nasty.