A good place to start is find out what the item costs new.
Have a look at

www.heliproz.com

One of the better sources for used heli stuff is right here on RCU.

The best deals are ussually pagage deals, where a guy starts out with good equipment and just quit. sometimes they sell off perfectly good and realativly new stuff for about half of what it would cost new. Be sure to do your homework. there is quite a lot of outdated junk floating around ebay.