My car needs a new radiator fan motor. Problem is, according to my mechanic, no one sells just the fan motor - they sell the motor + the fan assembly. And it's a spcial order part - to the tune of $480 for just the part.
Just a little searching around today, I found the part for under $200. And by a little searching, I mean 5 minutes with my favorite search engine.
So there's some problem in the car repair industry - car shops look at one or two sources to find a part - they don't hunt around for great prices. Since they mark up a percentage of what they pay, they have an incentive not to look around.
If they did look more, however, I'd create a network of parts buyers & sellers. Buyers get rare parts cheaper, mark them up, but pass on the savings to their customers - who are happier and more loyal to the intelligent mechanics who find cheaper parts and save them money. Sellers don't get quite so much a markup - but they sell their parts more quickly and do more business - if they play ball with prices. And it's easier all around.
Part of the problem, though, when I've searched for parts many aren't really findable without asking "do you have the anti-lock speed sensor for a front passenger brake on a 1990 Ford Thunderbird?" There needs to be a better, more specific part labeling system.
Moral of the story: I'm getting my car back from the shop tomorrow, about $600 poorer on a simple part. The market hasn't become very optimized. It's ripe for the picking.