General consensus on buying used eq.

First I like to thank everyone whois ever contributed in this forum. It has really lessen the learning curve in this complicated often times expensive hobby. Now that I know alot more and will be buying equiment to try out in my system I was wondering what in the general rules in buying certain equiment. Are there certain items that shouldn't be brought used? What other pitfall should I be aware of when buying used equiment? Thanks guys.