All attachments (and vehicles) can be sold back to the vehicle stores (including the quest-given "old" equipment) by bringing them to the sales floor area. Walking up to the thing gives an option to sell it. As you will no longer be able to move around something you've sold, be mindful of where you dump your old stuff, or you may have difficulty getting your new stuff out of the lot! Pick an out-of-the-way spot. Selling the old equipment is worth doing if you no longer need it; they're worth more than you might think (if still less than the modern equipment, obviously). As a side benefit, selling off all your unused equipment thins out that opaque mass of map markers around your homestead!