[Luke_James]

Do as above said. At the very simplest level, have the script check if the player has the weapon and, if they do, add ammo on to the gun for a lower price. If the player does not have the gun, he or she would then buy the weapon itself for a higher price.

A more advanced system would be to have the player buy ammo independently, regardless of whether he / she has the gun at the time, and then he / she can use the ammo later with a command.