How can A person sue you for something they bought for you?

You want to ask the question in Civil Litigation. I have never dealt with any issue remotely similar to this one and it definitely does not relate to intellectual property. My recollection from law school is that the law has a hard time making sense of gifts. That said, the law normally looks at who possesses a good and then tries to determine if that person either came by the good wrongly or owed it to someone else. If he wants the car back, his argument may be that he bought it for you to use, he did not buy it for you to have.  In that case, you will need to show that he intended for you to take ownership of the vehicle and not simply use and maintain it. The registration is strong evidence in that regard. Try someone with experience in civil litigation for a more knowledgeable response.