Carter was locked away at the NCR Correctional Facility after being framed over an undisclosed amount of caps by a pit boss while working in a casino in New Reno. During his time imprisoned, he was always known as the guy who could get things. This led to his career as a trader after the Powder Gangers took over the facility.
Selling an upgraded weapon to him, if he has the same weapon in his inventory, he will give the player character the un-upgraded version if bought again. It is not advised to sell any upgraded weapons to him.