It depends by what you mean by buying the devices online. If you have any Geotab serial number then you can add them to your database. I would advise however that you to obtain these devices through our official channels.
That will not guarantee that the devices will communicate to the database if they are used however. Devices that already exist in another database will continue to communicate to that database until they are either removed or made historical in that database.
Additionally the data that will be transmitted by the devices will depend on the plan that the device is currently under.
Let me know if you have any additional questions.