At your pricepoint, you're looking a white-label reseller; There's no margin at a full-sized server for 600$, so there's not a lot of interest in selling them.
I'd go back to why you believe you need a physical server and re-evaluate.
I'm ok with used, and the current suggestions have been more than helpful, although I took your advice and changed the price range to $700-1000.
You can get a Dell PowerEdge starting at $299 (no HDD). I have an older PowerEdge running Ubuntu and it's still humming along nicely.