I know that the xxD(40D,30d, etc) from Canon have approximately 100,000 shutter actuation life. The lower cameras do not show the actuations unless you send them into Canon. The replacement of a shutter is about the same price as a cheap used DSLR. I think the Rebel series are rated at 50,000, but I'm not for sure.
Aside from the usual cabeat emptor stuff that goes along with purchasing used complex electronics (not to mention eBay in general), any particular thoughts on wisdom of buying a used DSLR? I notice some listings on eBay mention the number of "actuations"--- is this a valid stat for gauging the use of the camera?