Some datacenter world lingo also helps:
With drive bays you'll see: SFF (small form factors, ie 2.5" drives) and LFF (3.5")
Usually written like "12 LFF" for say a server that can hold 12 3.5" drives.
Look out for if it comes with drive trays for the bays or not. They're usually surprisingly cheap because there's a bazillion of them out there but it could be $5-$20 each.
Then there's SAS / Raid controller stuff you need to look out for in regards to compatibility with your configuration.
For example if you're going to run TrueNAS (or whatever it's called nowadays) you'll want to search "XXXXX controller TrueNas compatibility" to stumble upon a thread or some hardware list to hear what people have to say. There actually aren't too many models, or they're clones of each other with a Dell or HP branding label on it, so you're bound to find some stuff, for mainstream stuff sold on ebay anyway.
The hardware will typically be at least power-on tested, which for this kind of equipment is usually good enough, but beyond that your experience will vary.
There will be typical gremlin stuff like an unseated cable which will throw errors and have you wondering if it's a bad drive, or bad install, or whatever... but I think if you're building your own and bringing older gear back to life, you won't be able to avoid that unless you're buying a pre-packaged NAS box.