Aftermarket barrel in a non-standard cartridge that isn't available from the factory will add a little, but that's about it. Custom paint/Duracoat/Cerakote are more of a personal preference thing and more often make a gun harder to sell than anything because you have to find someone who has similar tastes as you. IMO it's probably worth about what you paid for it new, maybe $50-75 more due to the barrel.