You are wise to decline. Can PayPal be used for firearms in the UK? Definitely not supposed to be used for guns here in the U.S.

The big problem is having a buyer claim the item is 'not as described'. The seller is liable for shipping both ways as well as a full refund on purchase price, and PayPal almost always sides with the buyer. Scammers return junk (literally trash), but they have a tracking number showing a package weighing x pounds was shipped back to the seller.

Do not have anything further to do with this guy.