I don't know the law very well here in TX, but just some generalities. If you have an ag exception on your land, you do not have to pay sales tax when buying a vehicle, or tractor, or trailer, and a lot of other things, when purchasing that item. You have to have a number assigned to your property before you can do this. On my trailers, it's a lower rate to register it every year. I did not do it on my truck because you can only use that vehicle for things related to your farm or ranch. The way it was explained to me, if I get pulled over and I have something in the back of my truck that is not related to farming, like a piece of furniture, or groceries, then I am in violation of the ag exception by using my truck for personal use. I use it for work most of the time and decided it wasn't worth the cost savings compared to the risk of getting caught and having to pay the fines. Kind of like red diesel. Lots cheaper, but don't get caught with it in your personal vehicle on the highway!!!