Windows has a variety of activation mechanisms. For each of them to work, particular data and possibly services needs to be in place. slmgr /ato will just trigger an activation, which is a process that, abstracted, goes somewhat like this:
- What activation mechanism is configured to be used?
- For the given activation mechanism, is product key and associated data (e.g. OEM license certificate) in place?
- Is the key and associated data valid? (Online checks, if any, happen.)
- If yes, activate. Activation is either permanent or periodical. OEM, MAK and retail is permanent, KMS is 180 days.
So what slmgr /ato will do in your case depends on what Windows is configured to do, which is what I asked you to check in my previous post (slmgr /dlv). What Windows will do by default depends on the installation media, and in your case, what the seller might have messed with on this particular installation media (you seem unsure whether it's a Microsoft pressed media or some small shop burned disc).
OEM factories do OEM_SLP activations. Along with hardware information, BIOS also contains information on what Windows versions the computer (or rather computer model) is licensed for. In Windows, OEMs install a certificate which corresponds with the mentioned BIOS data. Finally, they install a generic SLP key, which just tells Windows to use the certificate+BIOS data to determine valid/invalid license. OEMs install certificate+key (slmgr /ilc <cert>; slmgr/ipk <key>), then run slmgr /ato to trigger the actual activation checks, and on success the computer is permanently activated.
Running slmgr /ato makes sense for an OEM_SLP activation. But unless your computer happens to have the mentioned BIOS data corresponding to an installed license certificate (you may not have a certificate installed either), you can't do an OEM_SLP activation. You instead do OEM_COA activation, which means you just enter the key. You can do this by doing slmgr /ipk <key>, followed by slmgr /ato. Or enter the key in graphical dialogs somewhere in Windows, which will do the same stuff behind the scenes.
If you are doing an OEM_COA activation, you are supposed to enter your COA (sticker) key. If someone sells you a COA license and tells you not to enter the key, the seller doesn't know what he's talking about (messing up SLP/COA), or something is fishy. If you will tell us what Windows is configured to do, the answer might be more obvious.
If you own a computer that has a Windows COA sticker on it, and also has SLIC/SLP data in BIOS, you are fully in your right to use SLP activation instead of use the COA key. Which I personally find more practical, since I don't have to read off the tiny code on the bottom of the computer I am trying to type it into.