I think the general principle in a private sale is that the car should be as described. Quite how that bears on mileage reading accuracy I'm not sure.

Even if deemed not to be as described, it would presumably be a civil rather than a criminal matter (I'm not a lawyer, this is just making conversation).

The fact that he has divined this discrepancy using public information that would have been available to him at the time he bought it suggests that he did less than he could have done to mitigate whatever loss he would be claiming. That would probably limit what a court would award him, even if he were to sue you and if he were to be awarded anything at all.

If you have no money to buy the car back anyway then it is unlikely that he would consider you worth "powder and shot" even if he had a case.

Having said all that, I think he's trying it on. He's a trader, he's supposed to be an expert and should be able to check for himself that a car is as described. Even a private buyer has very little prospect of comeback on a private sale, a trader who should know better will get even less sympathy.

I would certainly tell him that you were unaware; that he obviously knew how to check the mileage anyway so he should have done so; and that you are in any case not in a position to buy it back. If he doesn't accept that then he will have to decide whether to sue you. I think he will decide not to bother.

You can easily check the car's MoT history for yourself - and if you have owned the car long enough for it to have had a test, the result of that will show the last 4 years mileage readings.

If you still have documents from your purchase of the car, proving innocence should not be difficult. I suspect the dealer just wishes he hadn't bought the car. Would a higher mileage make a big difference to its value? Probably not.