Military Flamethrower

My problem is the following: when someone wants to buy the Military Flamethrower, your suggested method (using military_flamethrower) works perfectly. However, if I want to set up a sale, it doesn't work. Even though the Flamethrower is in the inventory, it says "not enough items in inventory" when using your suggested name. If I use the traditional name, it just spams errors in the console.  I would like to make it so that players can not only buy but also sell the Military Flamethrower item.

Could you provide or suggest a solution for this?

Thank you in advance for your kind response.