This has the hallmarks of a classic scam. You lose your money and you lose your car. But if the purchaser pays you by cheque, once that cheque has cleared and several more days have passed (total 7 days from paying it in), then the cheque cannot be revoked and is a safer means of taking payment than by bank draft or transfer because they can be revoked after the event. You are correct that the new keeper has to sign the V5C, so that will become a matter of trust between you once the money is irrevocably received. Keep a photocopy of the V5C that you send him and then check with the DVLA that he has send the form to them.