on the same make of car (say 2011 lincoln town car limited, why is the market value different ($3,000) when the mileage on the 2 cars is within 2 thousand miles ?

All that is is the asking price. Negioate with both of the sellers, and check them both for any problems they don't dislose. A car's value is how bad you want it, need it, and the other way around, how badly the seller needs to dump the car or hold out. KBB lists are guides only, not gospel