LOL Don't listen to that guy. Don't add anything to it unless you are going to keep it for your own use. Adding mods to a weapon for sales purposes is a sure-fire way to piss your money away.
save the mod and just sell as is. charging extra for the mod will limit your base of potential buyers. Buyer A may not want Mod B so leave it as is and sell. You NEVER EVER mod a wep for sale unless you already have a specific buyer that wants it with a certain mod and the mod will actually make you even more with it then the price of the mod.