To be honest, I paraphrased the original conversation the seller and I had.
I showed him my ID, it's from out of state. Since this is a long gun purchase, that doesn't matter. He said no, I need to be a Texas resident.
I showed him a copy of my military orders making me a resident of Texas and my military ID. He said no, he's been burned by people showing Temporary Duty Orders. I helped show him how to read official Permanent Change of Station orders. I also told him that the largets military installation in the United States is 15 miles away, and there are lots of folks there who have disposable income. I can help him expand his potential client base. He still said nope, no military, only Texas State issued IDs.
I guess there's not a whole lot else that I could do to help show the guy how he could help his potential future business by understanding the papers uncle sam gives servicemembers.