Market vs Trade values

I'm interested in buying a pre-owned 1200GS, but I'm struggling to decide if the prices I see advertised on this and other forums are fair, given my lack of experience in bike-buying.

Where can I obtain the market and trade values for, say, 2006 - 2009 GS models?

Will BMW provide this information if I ask their dealerships nicely?

Or is there somewhere on the interwebs that I could look it up?

Any feedback welcome!