If you have a computer shop that is an authorized Apple reseller anywhere locally in your area, this would be a better place to purchase a used Mac. Authorized Apple resellers carry both new and used Macs.
There are, also very reputable Apple authorized resellers online that have very good reputations and guarantee the Macs they sell for good physical and working condition.
Reputable online Mac resellers will have pretty decent and extended warranty plans on used Macs and, also, offer support for their products.
Here are a few I recommend.
PowerMax, dvwarehouse, Used Mac Reseller ( these guys have a 12 month extended warranty on used Macs)
http://www.powermax.com/
http://www.usedmacreseller.com/SearchResults.asp?Cat=1828
http://www.dvwarehouse.com/Used-Macs-c-253_53.html?gclid=CIegoPTdqrgCFY2e4AodJA8 Akg
Good Luck!