For starters, I would go back to the county treasurer's office and try to get them to reimburse you. You might need to go above the person you've already spoken to. You do not own the house, you can prove that with documentation, you are in no way liable for the November payment.

If that doesn't work, you need to explain the situation to the buyer and ask them for help. They might say no, but then again they might reimburse you without hassle. If it were me, I would want to know so I can get my bank involved, because they should not be paying a tax payment that has already been paid. If they do, then once again the county has been overpaid. Also, I absolutely would want you to be reimbursed because it's just not right for you to pay taxes on my house.

I don't see how you can take the buyer to small claims court if you haven't tried other means to get the money from them.

Good luck!!