I've had great luck on ebay, if I know exactly what I want. You can find some very good HP Proliant servers on there, both newer stuff and older stuff.
I've been happy to buy new items off of ebay with it comes to RAM, but I do not buy used RAM if it can be avoided, unless I plan to test it myself extensively. There's nothing wrong with equipment that somebody else has used, so long as you ensure that it will not cause any issues by you using it.
Amazon and ebay have been good to me here as well, but I also will go to Microcenter if the price is right. The majority of the drives I buy are HGST 2TB HDDs, but I buy the occasional Silicon Power SSD or Corsair SSD from Amazon.
Note on used or refurbished items:
I've received items that were DOA when new, and I've had refurbs that outlasted some new items. So I tend to use battle-tested equipment when it comes to my own stuff. However, depending on the clients needs, sometimes the benefit of a support contract are worth it.