Instructions
Travel to a general store in the wasteland that sells and buys guns, ammunition and armor. Talk to the vendor.
Ask the vendor what they have to sell. Sell the weapon of your selected two with the higher condition, and consequently higher value to the vendor. Next sell the lower condition weapon.
Buy back the lower condition weapon, it should be the same price that you sold it for. It will be returned to you fully repaired.
Sell the weapon back to the vendor for the higher price applied to fully conditioned weapons.
Repeat these steps as many times as required to continue duplicating the amount of caps in your account.