Legally, in an "as is" private sale of a used car, you would only be liable for problems with the car if you knew of those problems (or reasonably should have known; i.e. any owner would have been expected to know) and did not disclose those problems. If that were the case, you would have committed fraud; fraud can provide grounds for the buyer to recover monetary compensation (such as the cost to repair) or possibly rescind the sale (return car; get money back) entirely. If you did not know of the problems, however, you should not be liable.