Probably $300-400 unless you have some fancy lighting to go with it. Unfortunately used equipment and tanks aren't really worth a lot of money. Lighting seems to be about the only thing that holds value. Figure $125 for the tank, $75-100 for the rock if it is in good shape and not full of algae (besides coralline algae), $100 for filters/heater/stand/salt.
If you can get more great, and the biggest factor is where you are located... but most folks list their setups for crazy high amounts on craigslist and they don't sell. You can't go by what it costs new... Start at $800 and take the first $400-500 that gets offered