You can get a new 100 watt panel delivered from solar blvd for $150. If you need a controller (not needed if you are going to be monitoring the system) for $15 on e-bay.
You can look on craig's list or e-bay, but I don't think you're going to do any better.
The price of panels has been dropping so fast, that I doubt you are going to find someone to sell a used one for less than that. A used 100 watt panel a few years old cost 200-300, most people would keep it, or not be willing to sell it for what its worth now.
Doug