These are from volume licenses. Not really legal but it mostly works in practice. In some cases the buyer gets unlucky and the volume key gets revoked due to too many activations, but either the seller will give you a replacement key, or it's just another few bucks for a new key.

What is a good way to check to ensure we are not being handed a key that will get revoked

Even in our markets, we have office licenses available on Amazon for all prices ...the cheapest ones looking very suspect but not so sure about the ones that cost a decent amount.

Way beyond my skills to say. I just use a reseller who’s been around for years and has decent trust pilot reviews. I wouldn’t use Amazon though!

CJS CD Keys. I used them before years ago for Visio 2019 and windows 10 pro. Both still purring away nicely