You can become an arms trafficker with Trevor after you complete the mission 'Nervous Ron' by purchasing the McKenzie Field Hangar. The hangar contains an airplane and a dune buggy which you'll use to traffic weapons by air and ground respectively.
Arms trafficking is a good source of bonus income as each mission pays out fairly well.
There are five air trafficking missions. To access them simply enter the plane in the hangar and you'll be given instructions by Oscar. Missions range from dropping off arms supplies to bombing rival factories, with bonus money awarded for completing bonus objectives such as finishing below a certain time.
There are five ground trafficking missions. As with air trafficking you simply need to enter the buggy to trigger one of the missions. These missions get you $5,000 each. The main difference between these and air trafficking missions is that you'll have to face competition from rivals.
|Tip: Don't stay on the main roads for too long as you'll raise suspicion and get a wanted level|