Personally I feel there would be a great market for the junkyard business on here.
I owned 5 older model cars in my time and I would often use the secondhand parts dealer when the car would breakdown. The thing here is that most junkyards are outside of city limits so you have to travel a clip to get your part. Also you usually wait till they take it off another vehicle or (as in one establishment) you could take it off yourself and they would give you a deal.
Who wants the hassel? A website would list what they have in stock, you click to order and its that simple. You wait for your part to be shipped.