If it was a friend of mine, and I honestly didn’t know about the radiator problem, and he came to me and said “You know, that sled I bought, the radiator had a hole I had to get fixed”, then I’d offer to pay for it.
In this case though it’s a stranger who’s first response is threatening to take you to court for something sold as-is. If it was me I’d honestly ofter to buy it back for the $400 he has proof that he paid you, then tell him to go fuck himself. I hate people who cry “I’ll sue you” like that.