How can I make sure it’s in 100% perfect working condition. ?

Memtest is what you’re after. You need to boot into it from a USB key because you can’t reliably test ram on a running OS.

There’s a commercial version that has a free offering or this open source one which I’ve found excellent.

I’d encourage you to sell it. In these trying times, any extra supply is helpful.

Yeah nazi Germany was around for 13 years ish. I’m not saying there’s no comparison, I’m just saying there’s no comparison, yet.