There is no registration in Texas. That paper you filled out at the gun shop was not a registration. It was to verify that you were not a criminal or had any other disqualifying conditions prior to purchase.
As stated above. Buyer gives you money, you give him/her gun. Done.
You are not required to inquire or ask any questions of the buyer, but should you already know that they are not able to legally own or purchase a firearm, it would be incumbent on you to not proceed.
I think he meant one of the UHaul Brigade of Yankees invading Texas over the past couple of years
The Law is that you can sell a gun to another resident of Texas provided he is not drunk and you have no knowledge of him being a felon
Always get a BoS, it offers absolutely no protection because it is not a legally binding document in any way and has actually been shown to get people in trouble, but it will make you feel all warm and fuzzy inside.