I've seen office professional for 15 bucks, I wouldn't say $25 for the basic version is a good deal… Especially considering you end up with a Microsoft product.
Those keys are mostly 'volume keys' which are meant for devs. Some people sign up as devs and pay some joining fees and in return they get lots of Office keys and sell it on their website. You can't use those product keys if you want to use Office on a different device as it'll not bind to your MS account or can't be activated on a different machine. You need to buy another key to use it on a different machine.
Well, depending on your use case, you might not care. If you want casual power point editing on your laptop, just slap a cheap key and it will work fine, buy another one for your home desktop and you are still <10% of a "genuine" licence. If you change your pc just buy a new one and you might even find a newer version. As for the MS account, I don't know anyone who uses it for office licence sharing, so I can't tell about it