Sorry to hear
Firsty why would anyone selling a car let someone drive it without them in the car....secondly why would anyone buying the car want to go for a drive by themselves ? Always get their drivers licence details, never hand over the keys and if possible get them to sign a documents stating that if they stack it while driving it they own it.
Cases in point...selling my first performance car (manual turbo mx6) a guy rocked up wanting to take it for a test drive....asked for his licence which showed he only had an auto licence...errr I think I'll take you for a drive!!!
Brother in law was a selling a motorbike....guys rocks up in a vintage monaro...as he handed the keys to him, he said "you stack it you own it".....guy comes back 10mins later with ripped leathers and a bent bike so as per the agreement he handed the keys to his monaro over to my bro in law and he took the damaged bike.