your location has to be taken into consideration...
here in the Houston area there is a guy that as a hobby/side business does exactly that...
he lists bike on craigslist almost constantly... he has a deal with a lot of local dealers who do not take trade-ins, when a customer wants a new bike, the dealers call him and he makes an offer on their existing bike, and then goes through the bikes and resells them..
I have not seen him selling anything less than about $1200 and occasionally he gets some really high end bikes. Most of the ones he lists go between $1800 to $2400.