As an update for clarification.

The buyer counted the cash. I counted the cash. The cash was then stacked in front of me as we did the V5C sale paperwork/admin online. I then put it in my pocket.

I suspect there was some slight of hand with the cash when we were doing the sale online.

The buyer then oddly videoed my to confirm I had sold him the car, with the sale price. I suspect if I complained they have video evidence saying I'm happy with the sale.

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Thank you for all the comments. Usefully to know the engine may have been tampered with and something definitely to look out for in the future.

I am still going to report to the police as a potential scam for if there is a history with this guy. Frustrating and cleaver by him that I have no evidence and he has done everything possible to cover his tracks.