Are you sure you tested it before sending?
I have just had a PM from another forum member, who was also interested in buying the gun, before I became involved.
He explained to me that he wanted to buy the gun, but wanted to test it first. He wanted to come to your place, test the gun, and then he would give you the cash.
You told him that you did not have any air to test it with. So, the sale did not go ahead.
Given this, you can do one of two things:-
i. Give me a refund of 50% of the purchase price - that's £90 - to reflect the fact it does not work, and was not advertised as such. I will keep the gun.
ii. Refund me the entire purchase price - £180 - and I will send the gun back to you, recorded & insured. Which means I will have taken a loss on postage and paypal fees.
Your choice - let me know which option you want to go with.