I've picked up quite a few lots locally off of craigslist, and yeah, you can get pretty lucky. I picked up a lot worth over $500 MSRP for a tenth of the price a few months ago, and the collection was mainly a bunch of sets (98-99% complete for all the kits).
If you can, keep CL open during the day, search regularly, and check the Toys & Games section.
Eventually, someone will post some, and if you can be on the ball, you might get a good lot
Just try not to pay an arm and a leg for lego without figs, and try not to pay more then $4-8 per pound (depending on content, and how bad you want it, of course) people tend to sell certain series for more, i.e. SW, western, and larger "vintage" 80-90's series.
I try to stick to $5/lb as a general rule for used lego (and btw, 1 gallon is approx 3 lbs).