The Novara is from REI and on their website it is new $800. But, used $600 is not a good price. Two years ago I sold a full suspension MTB that retailed for $1900. I got $850 for it. It took a few weeks to sell at that price also. Everything you linked to is overpriced in my opinion. Bikes on the Detroit and Ann Arbor CL are overpriced a lot of the time. There are a few bikes here and there that I feel are fair market value (my listings!!!), and a few under, but most are over priced.

There are deals out there, but I would start checking thrift stores and garage sales. I picked up two nice bikes today at two different garage sales in Plymouth.

Also, doing anything tomorrow? http://www.ann-arbor-bicycleshow.com/ Gates open to public at 8am $3 entry and you might find a decent candidate there.