Um, a TXDL demonstrates residency, that satisfies the law as far as I know as far as the seller is concerned.Let me let you guys in on a little story:
Back when the ATF was shutting down the Austin gun show they arrested a guy for "selling to a known illegal alien". This seller had asked the so-called "illegal alien" for identification and a Texas DL was produced. He then made the transaction, and was subsequently arrested and drug out of the Austin gun show, literally. It was a private gun sale from a private collection at a public gun show. He is now in the legal system trying to defend himself.
So a little cover your ass is in order from my perspective, especially knowing that story above. The degree of that depends on several factors, most of which is how well do I know this individual and is their any possibility that they might be excluded from the gun sale by any shape and form (prison tats, accents, etc.)
Law enforcement aside, the ordinary Joe would probably not be able to spot a fake I.D.. If it looks strongly like your D.L., what do you do?
Personally, if the buyer won't pony up a copy of the D.L. to be placed with a Bill of Sale, I don't sell. I also have them sign the bill of sale. Not a problem to date.