As for the receivers getting sold, yes activations live in the receiver. They can be moved/removed or for RTK transferred to the 7000 so you'd want to make sure they still have all the activations on it yet. Dealer can look up serials and make sure it has what is claimed or double check what seller assumes. 

If cell signal is an issue for you, sticking with the radio/base is good since it's working for you and you can do your own if your dealer ever stops their network. Might be handy to ask one of their techs where theirs are and how many you're using now to span your fields. 

It might be likely the onion farmer is going to 7000's and SF-RTK which comes from the satellites like SF3 does/did on the 6000. So as long as you can see the sky you should be good. No cell, no base station line of sight.