I like Amazon alright, but I love McMaster. Hope they can keep it going.
What happened to SmallParts.com? SmallParts, Inc., joined the Amazon family in 2005. In the past seven years, the dedicated team at SmallParts helped us expand our service and selection until we have grown well beyond our original focus of tubing, parts and fasteners for the medical supply and research industries. We have chosen the name AmazonSupply to help communicate our new, broader selection....
The website has a lot of relevant data that you need to make informed buying decisions. sometimes they even have cad file downloads with tolerances listed depending on the part.
Selling books/video online rather than in a store was a breakthrough area when Amazon got into it, ordering parts and materials isn't a new idea - Amazon is a lot less disruptive in this area.
My guess is that the biggest hit will be small specialist model/hobby shops, but unless you are in a few big cities there aren't any of these anyway.