How can I return a used car?

Your question requires an attorney consultation. It is not a simple question that can be answered on this type of forum. There are many factors that would need to be considered and evaluated. The short answer is that it will depend on how the car was sold and who you purchased it from. Was it sold "as is". Was it sold by a private individual or from a car lot. Since we do not have all the facts, it is impossible to give a completely accurate answers. I strongly suggest that you contact an experienced consumer rights attorney for a face-to-face consultation and give him/her all of the facts surrounding your situation. (S)he would then be in a better position to analyze your case and advise you of your options.

You need to find out what your state lemon laws are as to used cars [we do not know what state the car was sold in]. Immediately contact the seller and tell them what you found and what you suspect. How would you notice oil in the coolant unless you thought something might be wrong. You may need to find out what the other substance is as you may be wrong and the car is fine.

Do it now! Get a lawyer in case the seller gives you trouble.

Not a personal injury case. What does the contract say? Some state require a provision that allows a three day right of rescission.

Take it back to the dealer right away and see if they'll give you your money back or another functional car. Good luck.

What is the warranty, if any. Speak with an attorney, but generally it is now your car.