john mollahan

I'm new to Bike Forums with a few questions I hope someone out there can answer. I'm in my early 40's and began doing some light mountain biking about 7 years ago. Due to a bad knee, I can no longer tackle any rough terrain. I currently own a Trek 820. It's 6 years old and in great shape. It's 22.5 in. frame (I'm 6'3" tall). I'm curious about what the bike is worth. I do have some maintenence receips and the owners manual for the bike. I'd also like to know what the best avenue would be to sell it.
Thanks

John Mollahan