Buying a used helicopter can be a great way to save on the initial investment of purchasing a Radio control helicopter.  However you should take some things into consideration:

• Buy in Your Skill Level: If you are new to R/C helicopters just about any helicopter can accomplish training you. It is all in the Set-up of the helicopter.  A 3d aerobatic machine is not necessary. The older generation helis work great for this function. Their parts are a little harder to find, ebay is where most are found, but they are a little more stable . These heli's are the concepts DX, Nexus, Shuttle z-zxx , etc.
• Electronics( radio system): Older systems here are fine to if training. older rate and piezo gyros will easily get you through the forward flight stages and even mild aerobatics. The older radios may not be computerized but most of your time will be spent hovering at first anyways. If you own your own radio, transmitter, you may only want to buy one in which you add your receiver and battery back and set up on your system. Most new Transmitters have the ability to do both airplanes and Helicopters, check you manual.
• Condition of Helicopter: Look for damaged blades: Sign that a blade strike has occured, replacement of blades, main shaft, spindle,etc. Bent tail boom is a boom strike,replacement of Blades, tail boom, main and spindle shafts may be needed. Discolored Engine, dirt on the engine is normal ,if it is discolored it may have been run lean and overheated.It may need replacement or your helicopter may never run right. A reliable engine is key in good helicopter performance.
• Helicopter Class SIZE: The 30 size is your base(non electric) and are best for beginners. Airframe is smaller,easy to transport, and most parts are cheaper to replace. 45-50 size are the sport models and mid to advanced level machines. Mosta re highly aerobatic and capable of gravity defying maneuvers. a little bigger in size and price. 60-Gas sizes helicopters are large and are used for scale or aerobatics. More stable than hte other class but have higher mass and lots of power for aerobatics.These helicopters however tend to be quite expensive to repair if damaged. An average crash could cost you more than a 30 Size Helicopter. If using a 60 to train on take your time.
• Finally if a new pilot: SEEK HELP From a Heli Pilot, most are happy to help, the good pilots want you to succeed