Can't Sell Inventory Item in Defiance

Item needs to be unequipped from all loadouts. Review your inventory using 'I' on PC or pressing 'Start', then using the Right and Left Bumpers to navigate to the 'Inventory' tab. Here you can review exactly which items are considered equipped and which Loadout they are equipped to. If you make sure that any unwanted items are not currently equipped, you will be able to vendor any extra items as normal.