Hey guys! 
I wanna build a small home server for TrueNAS or Proxmox but not sure what hardware to start with. Like does CPU brand matter a lot, and how much RAM is actually enough?
Also is used server hardware from eBay worth it or should I just buy new?
Checking out these options:
https://www.amazon.com/s?k=nas+hard+drive+server+4tb
https://www.ebay.com/sch/i.html?_nkw=used+dell+poweredge+server
Any tips or build suggestions would be super helpful! 
Thanks!
Your question is really too broad to answer. What services do you want to run? TrueNAS and Proxmox are not equivalent/interchangeable software packages. TrueNAS excels as a data storage and protection server/Network Attached Storage (NAS). Proxmox excels as a virtualization host and is not designed to be a NAS at all. I actually run both in my environment. I have a dedicated TrueNAS box, stuffed with a lot of drives, and Proxmox box with just a few drives. TrueNAS can do virtualization and run containers, but I am not a fan of how they do it. I think their system hides to many details and you will not learn things like setting up and running VMs or docker containers properly if you go the TrueNAS route. Proxmox is not at all designed to be a NAS an people sometimes hack the software to get it behave like a NAS, but it is a Franken-NAS at best.
What services do you hope to run and what will be the purpose of your home server? All of that will dictate how much CPU power and memory you will need. Are you building this just to watch movies, or what is the ultimate goal?
My very first home lab device/server was a Raspberry Pi NAS with 8gb of RAM running OpenMediaVault. It only served as a NAS. I didn’t try to docker containers, mostly because I didn’t yet know how. I am now on my 6th or 7th home server, and have everything set up in a rack. Your needs will change as you learn and grow.
@emmaleo Emma, buy a Lenovo m720q with 16 GB, make it a proxmox 9. Buy another m720q, make it truenas. Use samba, mount servers so that you have storage. Slowly, over the years I have 11 m720q’s which I can repurpose easily, eg. add another proxmox node or cluster.
Start small, get your feet wet, then buy more h/w. No reason to start with a 24 TB NAS.