I find myself in a similar situation and the domain name is expiring next month I don't know what I should do.
Wait a couple of months and after that try to sell it again. It was buyers job to get the domain from you. If they have been stupid and forgotten about the domain they paid for then its their problem. Renewal is only 10 $. Right? How much you sold the domain for?
It was a site not a domain. It has been 8 months and they have not got back to me after repeated attempts of contacting them to transfer the site.