In most states a private seller of a used car, selling it to another individual, is obligated to do just one thing: tell the truth and not hide the truth. What the prospective buyer asks about, tell them what you know. Of course, it would be wise to tell them that you don't know anything else, so it's clear what the limit of your knowledge of the car's condition is. Whatever you do say, however, can amount to a warranty or guarantee so make sure you are accurate. You should always write out a simple handwritten contract too, or do one via email if that's appropriate, saying what is being sold (make, model, year, VIN# is enough), what the price is and what you (as the seller) are promising about the car, if anything, and both of you should sign it (an electronic signature, so to speak, via email is probably ok but check on what your state law says with a local lawyer). If you are selling the used car "as is" then you should write that on the sales contract or receipt too. As long as you answer questions truthfully and don't hide anything, then you should be okay if something goes wrong with the car later. HOWEVER, all the rules change if the seller is a car dealer (which in many states means only that you sell more than 6 cars per year). Car dealers almost always, in every state, have much greater legal obligations to the buyer, usually including: to sell only safe cars, to inspect vehicles before offering them for sale, to make written disclosures about any warranty rights, and a lot more. To find out for certain what your obligations are in your state, you should talk to a local Consumer Law attorney near you. You can call your local attorney bar association and ask for a referral or you can go to this National List of Consumer Law attorneys and find one near you: http://www.ohiolemonlaw.com/ocll-site/ocll-locate_local.shtml
In Texas, a private seller is bound by the Deceptive Trade Practices Act. So, your only safe harbor is to sell the vehicle "as is" with a written bill of sale which will relieve you of legal liability after the sale.