I would find a diplomatic way to state my position depending on the specific client but my thought is this: A "bid" implies specifications. There can be no bid without exact specifications to be bid upon.
I rarely have faced this situation probably because of the way I market my business i.e. that I am the best at what I do. That is the usual reason that potential clients seek me out.
Starry Night Lighting
"Always look on the bright side of life."