It's better to buy new Nerf guns instead of used. On used Nerf guns, it's possible the internal mechanisms that make it fire have broken or that the set is missing some of its darts.
When buying a used Nerf gun in person, a person can test it to make sure that everything is in working order. If the customer is buying online, he has no guarantees and may be unable to return it if it is defective. Buying new Nerf guns eliminates this problem. However, if a customer chooses to go with a used Nerf gun, Hasbro.com sells spare parts and ammo refills for Nerf guns.