Used enterprise gear: what's worth it and what's a trap
A decommissioned server is the best value in computing and one of the worst things you can put in a house. Both statements are completely true.
The rule: work out the three-year electricity cost before you look at the purchase price. On used enterprise kit, the sticker is usually the smaller number.
What's genuinely worth buying used
RAM-heavy servers, if you are learning. A used PowerEdge or ProLiant (Amazon search, opens in a new tab) gives you out-of-band management, hardware RAID, redundant PSUs and more memory than you can fill — the exact things a career in infrastructure runs on and the exact things you cannot practise on a mini PC. If the goal is skills, nothing else is close.
Small-form-factor business desktops. The best-kept secret in the hobby. Three-generation-old office micros (Amazon search, opens in a new tab) sell for very little, are quiet, and make perfectly good Proxmox nodes. This is the used-gear buy with the fewest catches, though older platforms still idle higher than a current mini PC.
Enterprise drives — with care. Used drives are fine as a second copy or scratch space. They are not fine as your only copy, and a drive with 40,000 power-on hours is a drive on borrowed time.
Managed switches. Enterprise networking gear is durable, cheap used, and features do not rot the way CPUs do. A managed switch (Amazon search, opens in a new tab) bought used will outlive several servers.
The traps
Noise. A 1U server at boot sounds like a hairdryer and never fully calms down. You cannot fix this: 40mm fans at 8,000rpm are the only way to move air through that chassis, and quiet replacement fans (Amazon search, opens in a new tab) only help in a tower or 2U-and-larger. Every "I put it in the loft" story starts with someone underestimating this.
Power. Older Xeons idle where a mini PC peaks. Depending on where you live, running one continuously can cost more per year than the server did. Do this arithmetic first; it decides the purchase more often than any spec.
Proprietary parts. Vendor-locked caddies, PSUs and risers turn a cheap upgrade into a scavenger hunt. Check what a spare costs before you commit.
Firmware behind a support contract. Some vendors put BIOS and management updates behind an active entitlement, which you do not have. Research the model, not the category.
The picks
Best used buy for almost everyone: a small-form-factor business desktop. Cheap, silent, and it makes no demands on your house.
Best used buy if you are studying for work: a 2U server with IPMI — 2U, not 1U, because 2U can be made quiet and 1U cannot. Put it somewhere with a door.
Best used networking: a managed switch. Almost no risk here.
Do not buy used: the drives holding your only copy of anything, and any 1U box that will share a floor with a bedroom. Buy a UPS (Amazon search, opens in a new tab) with the money instead — it protects more than the server does.