It depends on what they are. If they're clothing/things that touch the skin or hair, you're good. Most of the stuff you get to keep since the military can't reuse them. IOTVs, Goretex, helmets, all are to be turned in when clearing out of the military. If he still has them after he cleared, that means either he stole them or they were issued to him but not put on his clothing record. Either way, they are still government property even if the the military never asked for it back. I have a DCU IBA and plates plus my ACU IBA and plates because I was issued them, but the DCU one is not on my clothing record. Hence why I received a second IBA. When I clear, both will be going back to CIF. I only keep the DCU one around so FNGs can use it if we happen to go to the range before they get their TA-50 issued.
Bottom line: if you want to buy it, go ahead. If you get caught with it, you'll have it confiscated and you'll be out your cash. Or, you could always call your local Provost Marshall and let them know someone is trying to sell government property.