Josh,
You could post the question here
http://www.ar15.com/forums/forum.html?b=8&f=8 and you will get your answer quickly by gun people who live in Texas. You could also ask at a good sized gun shop. SOMETIMES they know the law.
It matters what state you are in and many times it matters whether you are talking about a long gun or handgun. It also matters if you are selling face to face or over the web as well.
For example in NC as a private individual you can legally sell your rifle to another private individual (as long as they can legally buy one) without any paperwork. However, to be strictly legal in NC the buyer of a handgun MUST have a purchase permit to buy from you. Trying to do otherwise, say in the parking lot of a NC gun show, and you WILL find yourself charged with a felony. Something that wouldn't go over well in Kingville.
It can get complicated so be sure the answers you get are Texas specific.
PM me with what you are selling? I might be interested..