Like any online store, Amazon is great if you know exactly what you want, but where traditional wholesalers have the advantage is their knowledgable sales staff. OK, I know how sarcasistic that sounds, but there is always that one person at ADI and maybe two at Tri-Ed who have worked their long enough to offer advice. Imagine calling (emailing?) Amazon and asking what device you need to allow you to power a 30watt PTZ camera over 400m away? Or asking what credentials are compatible with a specific access control unit.
Now if Amazon creates a "systems group" that can answer the phone and answer these questions and commits to it long-term, they could really hurt traditional distributors.