Craigslist is my best source for deals, although I have found one really great buy here on RCU. The best buys are the ads that go something like this: "several RC planes, a radio, and a whole box of accessories" with maybe a couple of fuzzy pictures. The seller often can't tell you exactly what planes and engines he has and doesn't care enough to research it for you. You can make a lowball offer based on what you can tell from the pics and about half the time they'll take it within a week or two. You can usually be confident the airframes are fine and the tools are fine. Engines may need a cleanout and new bearings but they are generally fine too. Don't expect any of the batteries to be any good and figure on 1/3 of the servos to be no good. Don't pay anything for old analog radios, but a nice computerized one can be resold.
To give an example, I offered a guy on Craigslist $150 for a lot of 3 planes and engines and a box of "tools and stuff." He said no, then came back about 6 weeks later said no one else had even showed interest and accepted. The ad had one wide angle picture of the lot spread out in his yard. I called him to arrange to meet, and he offered me his old radio for $75. I didn't need it and told him he could probably get $50-$60 on Ebay. He didn't want to bother with that so he accepted my offer of $25 for it. So for $175, I got 3 .40 size planes that needed some basic repair and covering work, a charger that he didn't even list that I sold for $50, a radio that I sold for $75 (surprised me), 3 engines including two .25's that brought $70 on Ebay and a nearly new TT .46 Pro that now resides on my Kaos, and a whole box of tires, tanks, props, fasteners and linkages that will save me probably $50 in hobby shop visits over the next few years. So now I'm about $20 ahead and have invested about 4 hours and some gas between traveling to buy the lot and listing the items I didn't need.