In a situation like this I would normally recommend good communication and make every attempt to resolve any misunderstandings with the buyer HOWEVER this buyer sounds uninformed and seems to have a bad attitude so I would suggest not attempting to force the deal because it will be too much trouble, frustration, and not worth the effort.
I would recommend, possibly after giving the buyer one more chance to respond, filing a non-paying buyer report, relisting your gun, and giving the buyer an F for Failure to payer or use a brain. I think you might be opening yourself up to heartache offering the single action conversion suggested previously but if you do and it works out then good for you. Good luck. Sorry for your troubles.
Ironically I was reading a surprising article this morning that leaves me even more dumbfounded in the context of this thread. Apparently you cannot sell, or it's very difficult to sell, an old .22 LR revolver to someone in California BUT according to Janie Harr, Associated Press, in the San Jose Mercury News, "After a new law went into effect in January, more than half of all new driver’s licenses issued in California this year have gone to people who are in the country illegally, the state said Friday."
Confused look on my face ...... fingers scratchin' my head.