Sorry, I think you’re well off course. Legally the police won’t be interested - the duty lies with the driver.
Under a civil action the onus of proof is less but still your advert will match your invoice. The only argument would laughably be about allowing someone to drive away in THEIR car but how on earth can we stop someone driving away in THEIR car? Sorry but that’s a load of old tosh, if someone stabs someone does the victim take the retailer who sold the knife to court?
Good luck refusing to hand over the keys to someone you’ve just sold a car too. My responsibility is to advertise correctly, not mislead customers & record the sale correctly. I’m not a moral guardian for the actions of strangers.
If you think something through to the worst possible scenario you’d never, ever sell anything.