Can someone Sue me and win, because i sold them a car with problems?

Ok so about 6 months ago, I sold a car to a friend for $4,000. the car had a previous history of an accident, nut no history of mechanical errors. I fixed the car myself and sold it to them telling them that it didn't have any problems, only that it needed tires and an alignment and that i'd try to help them if any problems occurred. At the time of the sale, I told them to test drive it and they didn't because they said they trusted me. 2 weeks later i get a complaint saying the brakes are vibrating. Right there and then, I Fixed the issue 3 months later something happened to 2 of the wheels and they were wobbling. their repair costed them 500$ and want me to pay for it because they accuse me of lying to them. when they bought the car I signed a receipt but only a receipt. since I refused to pay for it. 6 months they cent me a letter saying I either take care of the problem, or they want their money back and return the car to me. attached with the letter, they sent me a copy of the receipt I signed and with it, an attached "purchase Agreement" stating i'd fix EVERY problem that arises with the car. the purchase agreement isn't signed by me or dated.and wasnt present at the time I purchased the car. can they beat me in court?