Many repair shops make all their purchases off eBay. Not simply odds and ends, but everything. Sometimes, this works. Sometimes, an evil parts vendor sends you broken crap, which you must waste a week or two on returning, as your customer rips you a new one.
The answer to retain customer satisfaction is simple; stock parts! At the very least, stock parts for basic repairs. For example, customers should never be waiting on parts orders for simple iPhone screen repairs if you are an iPhone repair shop.