Welcome to the group! You'll find a wealth of material here.
As a bit of friendly advice (and I mean that - I want you to have a good time here), try to search for what you're looking for before posting. Most questions have been answered before, and at length, by thoughtful, knowledgeable people, and you get your results immediately.
But to answer your question, there are subforums for members selling a bus
and for non-affiliated buses
we've found. Off-site, the favorite places discussed are GovDeals.com
. And, of course, there's Craigslist and eBay.