I have been looking for used bikes on CL, but mostly only recent/modern bikes/frames. I'm not sure what to look for when it comes to vintage...
I'd say anything made of Reynolds 531 or Columbus tubing should be nice and light. A big popular choice is the Raleigh International, but don't set your heart on any particular model. In fact, I know a guy selling a 56cm International fairly cheap. I don't remember how much, but I could find out if you like. What size are you?