If your car is in good running order any issues the buyer brings up about the car can easily be discussed, especially if your and buyer mechanic have looked at the car. If you stated that the car is to be sold "as is" in your ad, refer to that while discussing this matters at hand. If the car is very old, you must understand that it will be more likely to have problems, if the buyers mechanic finds a huge amount of problems, accept what you seem is fair, but draw the line when it's getting to close to comfort. You can adjust the price to make up for the issues but don't let the buyer scam you, make sure you know what you are and are not willing to take.