I used to buy direct from SuperMicro distributors. I have bought from eBay and through amazon stores too. It just comes down to what you need and if the system is prebuilt with burn-in time.
I can't recall the place we used to purchase in Santa Clara, CA, (i thin it was acmemicro) but I would drive from Southern California to their place (about 6 hours) and would buy 20 1u servers at a time.
I've purchased thousands of gigs of memory through Amazon, and also purchased hard drives in bulk (each server we had was configured in Raid5 or Raid10, so that was 3 or 4 drives for each device, and alter Raid50 (6 drive systems).
Buying direct from Dell is a great option if you can get the price low enough, but if your buying just a couple of machines, it's often better to even try your local distributors and ask them to special order the systems for you. Usually, their bulk price is going to come out lower then you can get going direct.
Also, don't forget the place where you're putting the hardware. Datacenters are usually partners too, and many of them will let you purchase their equipment or they can purchase on your behalf.
Side note: If you're in the Omaha Nebraska area and want some of my old equipment (P4's and the like), come by with a truck and take them away. I think I have 50 of them sitting in the garage
They are older equipment however, machines that haven't been used since before 2008 and costs me more money to get rid of them than to just store them