The portrayal of a USMC reservist is inaccurate in several regards. First, the haircut; no Marine, even a reservist, would have hair that long. Second, Marines are not permitted to wear their camouflage uniforms in public, and wearing just a T-shirt without the uniform blouse off-base is even more unlikely. Even a rogue reservist selling black-market arms/ammo would not flout regulations in this manner as it would draw undue attention. Finally, there are no 'quartermaster' units in the USMC, making his cover story even more unlikely.
After Paul Briggs and Mike Warren leave the warehouse and are driving in Paul's Jeep, police lights are reflected in the Jeep's bumper.
The salesclerk tells Mike that Paul payed for the GPS device cash. But Mike traced the store through Paul's credit card history.