See thats not a bad idea at all! maybe that should be a project that some us could start together? ... but then again, theres lots of replacement parts on thingiverse.
But i believe that people dont actually upload all of their stuff, as they dont get anything in return.
I think i wouldnt use the service myself, except for very complicated parts as i myself am pretty good at designing parts from scratch, but others may not be as good at it as the next guy.
Anyone that would be interested in this kind of service, should reply to this thread. as i might be willing to participate in managing such a site