I would imagine that the reason they don't give private party figures is that they have no releiable way to collect that data.
I would also recommend doing some Googling to find an Internet forum/group for the particular bike you're interested in and ask there what the price of a bike in xyz condition should be. They should also be able to give specifics on what to look for in buying that bike.
93 BMW K1100LT "The Green Hornet"
91 BMW K100RS 4V "Kato"
IBA #17739 (SS1K, BBG, 50CC)