Click here for the complete rundown of how to link your cartridges.
You'll find that some people link, and others don't. If you buy used cartridges off the internet, check or ask if they are linked or non-linked. Remember... linking is permanent! Once a cartridge is linked to your account (regardless of which method you chose) it cannot be unlinked or linked to another account. If you buy used cartridges that have been linked you can use them in the machine only! If it doesn't specify, ask!
I hope this helps!