There are two websites that sell secondhand bikes that I know of
Hobart wheelers and Dirt Devils has a forum where people can post bikes to buy and sell. The following link will take you to the forum: http://www.dirtdevilsmtb.com/forum/viewforum.php?forum_id=14
There is also a website called Pedalbite (www.pedalbite.com) which also has a classifieds section in the forum, you can check the location of the bike when viewing the post.
Most bike shops also sell second hand bikes if you go in and ask they will show you what they have. almost 10 years ago