I'm not going to buy a car after I take this book. What if my parents insist on buying me a car? I have no money to pay my parents back.

There are two points in your statement that need to be analyzed. One is "What if my parents insist on buying me a car?" The background of this sentence is that parents want to buy you a car, but they don't necessarily buy you a car. The second sentence is "I can't pay my parents back". This sentence is difficult to understand. Since your parents want to buy you a car, it's not usury. Why can't you afford it? Besides, if they buy you a car, they may not want you to pay back the money in the first place. Your parents spent more money raising you than buying a car and didn't ask you to pay back the money. So don't think too much, just let nature take its course.