The simple answer for tight wad HO's is "We deliver the part, install it, and more important we warranty it. See if Ebay's sellers will do anymore than deliver the part.
I wouldn't be interested in installing parts provided by a HO. If they bought the wrong part or misdiagnosed the problem they might not be happy paying for the service.
The service industry can partly blame themselves. I don't remember this being a problem before flat rate and other excessive profit motivated services became common.