I've purchased almost everything used except for the case (using a Fractal Design Define 7 XL and Fractal Design R5).

Used/retired enterprise hardware has virtually zero market, so it's dirt cheap.

I'm using Dell Precision motherboards and Intel Xeon CPUs in both of my main production servers because each motherboard/CPU combo ran about $30 each.

For hard drives specifically I almost always buy used, but I always buy enterprise/NAS drives (HGST DC, Seagate Exos, and Western Digital Red Plus to name a few).