Depends very much on the make, type, condition and anything added onto it (eg sights).
Airguns can vary from a few quids’ worth of chinese scrap to several thousand pounds’ worth of elite target rifle.
If you need any help identifying it post up a pic or read the maker’s name and calibre off the rifle (usually stamped or engraved on the side)
The options are a) hand it in to a firearms dealer to have it scrapped (you can always ebay the sights, slip and anything else, just not the rifle)
b) sell it via Gunstar, Guntrader or any other forum in line with their rules. Face to face is preferable, as the rules around posting make ParfelForce 48 about the only way to do it properly. Read their rules before paying for the service!