File your own claim with paypal and the issuing credit card carrier.
Do a WHOIS search and see where the domain name is.
Contact your losing registrar and gaining registrar IMMEDIATELY. Give them the straight up facts of the domain name, sale, PayPal transaction ID, the PayPal memos, etc.
Most registrars also have legal departments (ie, GoDaddy's is
[email protected]) that are accessible.
CALL both registrars, get names of people you speak with, and follow this up with the material you have to present. Get confirmation from each registrar regarding your filing an action or a dispute.
Minimize any personal comments or opinions. They like the facts and can take action quicker without weeding through the BS.
If this was fraudulent use of the Credit Card, then contact the credit card holder.
There needs to be a police report made out and you will want a copy of that.
Have the CC holder provide you with a CC transaction and claim number so you can contact the Credit Card issuer and ask for their report and findings.
Contact your local law enforcement authorities to file a police report because you are now a victim of fraud.
You can also report this to the FBI internet fraud:
http://www.ic3.gov/
http://www.fbi.gov/cyberinvest/cyberhome.htm
and other Department of Justice branches:
http://www.cybercrime.gov/
So, you were the victim of fraud. But here is the kicker...
...how do you know that the fraud is not taking place now?
In other words, give the CC holder the benefit of the doubt. But at the same time, I would be suspicious of the "after the sale" fraud. Following these actions will help resolve two issues:
It may help you get the domain back and...
It may (or may not) validate the claim of unauthorized CC use.
Again, give everyone the benefit of the doubt. But if this was a bogus claim then that person is in a heaping pile of trouble once you file the necessary police reports and fraud actions.
Good luck!