I don’t know where you are nor your local market condition, but I’ve been checking GS500 prices on Craigslist in the SF Bay Area for about the last 3 years. I feel $2,500 is good and fair for an ’06 w/ 7k miles. For an older model (like an ’01) with comparable mileage could go for less than $2k, if you can find one. I recently saw an ’01 GS500 w/ 5k miles listed on Craigslist for $1,500; that was a rare and crazy deal, and it was sold within a day. A year ago, I bought my ’01 w/ 8k miles for $1,750.
In this down market for used bikes, I’ve seen some rediculously low asking prices recently on late model, low mileage bikes, for which the owners seemed just wanted to get rid of the bikes. Those “abnormally low” asking prices tend to depress the resale prices across the board.