Ask this question in the Black rifles section, and put up a couple decent pics, along with details on overall condition, how it was deactivated etc.
They'll be able to give you a good estimate of its value.
While they're replying, apply to the moderators for Equipment Exchange (EE) access. That is our "buy and sell" section that you'll be able to list it in. There's a little tab near the bottom of the forums page that you click on and it explains it. It's a little hidden. You can also read through the Newbies section right under this welcomes section and it explains it there.
Welcome!