Here's a more specific concept for the buying missions:
Every 3 missions, there's an arms sale in some remote location. Each of the terrorist bands is allowed to send one representative to the secret sale. You load up a truck with all the spare guns that your group has "acquired" ;D, then drive to the place. At the sale, you can walk up to any one of the representatives and when you get close enough to the representative six new actions will pop up like this:
Buy Weapons
Buy Ammo
Buy Other
Sell Weapons
Sell Ammo
Sell Other
Once you select one of these, these actions will disappear. When you select one of the buy options, you'll get a list on the action menu of what is available in that category and once you select a specific item, it's added to the guns in your truck. The "weapons" and "ammo" categories are obvious, and the "other" category will have stuff like NV Goggles, Mines, Satchel Charges, etc...
When you select the "sell" option, you get a menu of items that you can sell. You can sell anything that you're carrying at the moment. You'll get some cash for the item, which will now appear on the seller's "buy" menu.
The Buy menu will be easy, but the Sell menu will probably be much harder to make.