Car sales problems!

Dear engine-talk community. I had bought a car about a month ago to get to university over the winter. Was a small nice car with rest-tüv, rear an accident damage (from the outside) repaired, which did not disturb me, because the car drove perfectly and from the outside was top! The accident is from the previous-before-owner! So over 2 years ago. However, the way is very far to the university, the fuel consumption over the long distance too high. Now I sold the car 2 weeks ago, but with Sooo now to the important part of the sale. Actually great worked out, nice contact. The first time the father and the daughter came by. Have looked at the car and made a test drive. The next day the daughter (buyer) and I met at the traffic office to log off the car and re-register and then conclude the purchase contract. In the purchase contract I wrote as a special agreement: Purchased as seen, without guarantee (because I can drive the car). At the check mark: “Has the car in your possession suffered an accident damage”, I checked nothing. At the check mark: “Is the car accident free in the rest of the time, according to your knowledge”, I checked nothing, since he had once got one on it years ago. When the buyer looked at the car again at the traffic office and examined the trunk she said: “Oh that’s quite dented”. Then I said: “No Ah! I have tried to find a solution, of course, but you know every conversation. Well, I’m afraid as a poor student :P, of course, it’s not from my property.” She didn’t say anything else and she gave me the money. Two days after the sale, the father contacted me and said: “We’ll show you now, since the car has an accident damage in the back, economic total damage, you meant the car is accident-free (although I never said that!)”. Can you kindly give me information about who is right? I know that a lot of people will be brought to your money by fraudsters, but on the other hand I find that people who buy cars should make sure that the car is the right one and not simply demand a return, just because the car does not come smoothly through the TÜV. What do you think? Thank you in advance!