Buying a used bike can be frustrating. However, a reputable bike shop is probably the safest place to buy one. The best used bikes I've seen were sold by a reputable shop that sells that brand of bike and took the bike in on a trade-in. Typically these bikes are late model bikes. If you're looking at a used bike from a good bike shop, they should be able to go over their check list of what they reviewed when the put the bike up for sale. The better shops warranty the bike for a reasonable period of time. The downside is, they rarely have a large enough selection to put you on the right size bike in your price range. Still, don't give up on the bike shop possibilities, just shop around. Keep in mind, the used bike industry is huge. Estimated sales are at $1.3 billion per year (figure provided by the retail bicycle industry web site) and that does NOT include private sales. So there's a lot of shops flipping used bikes.