I wanted to put up a quick post on purchasing a used bike. It can be intimidating walking into a bike store, only be to shown $5000 bikes. Seriously....you could buy a car for that! Good news, you don't NEED to purchase a $5K bike to get a good bike and get out and enjoy some trails.
First off, I DO think it is a good idea to head into your local bike shop and test ride a couple bikes and get fitted to a frame. All bike shops will allow you to take the bike out for a quick test ride in the parking lot. If they won't, leave. Once you have an idea of brand, model, frame size and options that you would like to see on your "new to you" bike, I suggest the following sites.
I have not used this site before, but I love how many options you have to look through! This is why having an idea of brand and options is important. Check out PinkBike. It's an online used bike store, plus a whole lot more.
Next is CraigsList, but as always, with Craigs List....buyer beware! Please take someone with you if you are going to check out a bike, just for safety reasons. But also a second set of eyes on the bike.
Here in Denver there is a FaceBook page that is pretty sweet for people looking to buy and sell bikes. They are 14K members strong. You can post what you are looking for or keep an eye out for someone selling. Check out Front Range Bike Swap on FaceBook.
There are a couple Used Bike Stores around town too that are worth checking out if you have the time.
Randy's Recycled Cycles - I haven't been to this store yet, but I think they would be worth checking out as well.
Here is a quick video on what to look for when looking at a used bike. It is very informational and will help you out when it comes to making your final decision!