I'm transfering two machineguns to another individual in Houston and there is no sales tax implication. Now if the BATFE just didn't take 6+ months on the paperworks, we could get on with this. When I did purchase a machinegun from an FFL in Dallas, that was a different story.
As long as you are not making it a business of selling items. Texas allows you to have one garage sale every quarter. Any more than that and you are operating a business w/tax implications.
Used to have a Reserve Deputy who was a full time employee of the Comptroller's office. He would pass out sales tax use permit applications to the firwood salesmen on the side of the road if he saw the same person twice. They are running a business.