Well, if you knew about the leaking radiator beforehand, the ethical thing to do would be to pay for the repair, especially if you represented the sled in perfect shape. If you were unaware that it was leaking, but didn’t know either way, maybe offer to go halves on the repair. If you know it wasn’t, I’d tell him sorry, get lost.
Realize though that if you pay to have this fixed, the guy isn’t going to leave you alone with every other little problem he comes across. But on the other hand if he does end up taking you to court, even if he doesn’t win (probable outcome), it would still be a huge pain in your ass.