If it was paid via PayPal I would open a case against the seller. Just don't tell them an air pistol was part of the sale. For future reference the ONLY legitimate way to post airguns, rifles or pistols, is via ParcelForce48. Don't pay for extra insurance as the limit for airguns is £100. You may get resistance from Post Office staff saying they don't accept weapons. While this is correct there is an exception that allows for low powered airguns, it's in the terms & conditions leaflet. Don't use RMSD as that doesn't allow airguns. In fact as far as I am aware none of the courier companies accept airguns. If you use their services & the goods go missing you won't get any compensation, if they discover what is being delivered they will destroy them. Airguns coming from/to Scotland have their own problems due to their licensing laws now in place.
I hope you parcel turns up, fingers crossed for you.
UBC Resident Cowboy
St Paul of 55
Been there, bought it, tried it, sold it