Legally the standard is "knowingly". You cannot knowingly sell to a resident of another state.
Knowingly is a culpable mental state, and is defined. It is a higher standard than "reckless" or "with criminal negligence".
If the guy has out of states plates, but tells you he is a Texas resident, then it might be reckless to sell to him, but not knowingly.
With all of the people coming in from commiefornia and other broke states, if they have not changed the license plate on the car, I would not sell to them, for that matter I would not buy from them.