Is there a law covering used car purchases?

There are very few purchases that are covered automatically by the three day right of rescission laws under the Truth in Lending Act but consider that if the contract actually embeds a three day right of rescission, then you need to abide by that agreement. As to lemon laws covering used car purchases, checking with your state's consumer protection agency would be able to tell you whether your state allows for lemon law complaints for used cars and what those criteria are to qualify.