i'd be a little leary about buying a lightweight used frame. most lightweight frames are made of aluminum which doesnt have an infinite fatigue life, so if you buy something used who knows how long it will be before the frame breaks.
there are plenty of brand new frames on the market that are light and are $300 or less.
http://www.bikeman.com/KON-F09KU17.html
http://www.treefortbikes.com/338_333...One-Frame.html
http://www.leaderbikestore.com/pd_ld_516h.cfm
http://www.performancebike.com/bikes...00_20000_64503