If it's taxed he hasn't got a problem. there is enough evidence to prove when he bought the vehicle.
I am beginning to think this is a tax fiddle on the part of the the garage owner, obvs he'd make a profit on each sale and it is this that he will be taxed on. If he can say he sold the car on a different date from the one he actually did then there may be a reason for it like he is trying to avoid paying tax on the sale in the month or year in which the sale took place. He prolly only passes papers on to his accountant once a month. This may be what he is referring to. I am not an accountant so I have no idea, but this is the olny thing that makes any sort of sense to me, and that isn't much sense at all!!!