Ebay, Craigslist, Local Pawnshops (sometimes), college posting boards. If you're in a major metro area craigslist can be very helpful. Ebay tends to bid pretty high and you can't ride or see the bike before you get it. Our local University often has bikes posted for sale. Big variety.
I have the used success story of all time, I think: I dropped by Rivendell one day to check out the new "retail space," and while I was standing there, they rolled out a used Rambouillet, 64cm, built up with 105 components. It had been a shop demo. I got it for $1000 with new Ruffy Tuffys.
Three of my other four bikes also were bought used--an Allez and Trek from newspaper classified ads and a Hardrock my wife uses for a townie from a local thrift shop. Prices ran 40-50 percent of new ($10 for the Hardrock), and I haven't had problems or surprises from any of them (I'm only a so-so mechanic, but bikes are pretty easy to check out).
The downside is that you may have to look a long time to find what you want, especially in a higher-end bike and/or an unusual size. There just aren't that many 64cm bikes out there.
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