I've sold lots of my personal bows on an archer site and one air rifle here. I agree, forums like this one are great places to find buyers who know what they are getting.
Take some good pictures showing the gun's best perspectives and condition. Pictures shown in action (outdoors or indoors on shooting bags, etc.) help because they inspire the buyer's imagination.
Tell all about the gun that you can. Are you the first owner, how much used, how well cared for, any shooting specs (accuracy FPS). Any additions, improvements, issues, or repairs you've been through.
Finally, a fair price and clarity about how you want to be paid and who pays shipping. Doesn't hurt to share what you've paid for what you are selling to emphasize a good deal.
Good luck.