I am not so sure an online supply house is the direction you want to go, there is a lot to be said about brick and mortar service, especially when you need you stuff NOW, or you need to fix a problem.
Any advice I can give would only pertain to my local supply houses, and a supply house is really just a reflection of four people, your outside sales guy, the inside sales guy, branch manger and the credit department rep. If any of the four don’t know their business, you will pay more. Some supply houses like to double bill, watch for it. Also if a supply house plays game with lien releases, drop them fast and never go back, they can put you out of business. Anything you tell your outside salesman will be repeated to every competitor you have, use this to your advantage.
Go to your local supply houses, meet the inside guy that does the quotes, build your relationships from there. Try to quote stock and job orders to at least three supply houses and NEVER NEVER NEVER EVER, shop or share their numbers, in the long run it WILL hurt you. If you get a reputation for doing this you will never get the right price.