Ebay won't unless you split it into components.
Airgunbbs is a reasonable site to sell on, not sure if you have to post on there to get access to the sales section or not?
Freeads I believe do allow or certainly used to allow sales.
Other option find a local club, see if they have a for sales board and add it to there if allowed.
Finally go along to your local gun shop and ask them if they'd buy it, but be prepared for a shock on what they offer you.
