How to sell items marked as sellable?
Solution 1:
Somebody asked the same question here.
When interacting with a vendor press the on-screen prompt to switch from purchasing items to selling your gear (Default Square PS3/X X360) and then use the bumpers on your controller to switch categories of gear you can sell (Default LB/RB). This allows you to sell supplies, gear, or ammunition.
Refer to Rage's wikia page on vendors for a list of people you can sell gear to and what gear is safe to sell.