I didn't say that any voluntary transaction is legitimate. Rights can be a complex issue, but your example was solved long ago in several ways, such as with easements.
You won't be able to disprove the legitimacy of the concept of rights with a trite example, it would require a more serious discussion than we'll have here.
Two groups of cell towers using the same piece of spectrum interfere with each other in the same way as those walls.
The fact you don't understand what the walls scenario is identical to what you are stating is okay is depressing.
It is only when anyone is able to produce a good and distribute it, given sufficient capital, that your mental model is appropriate. No one can simultaneously control the same spectrum in the same geographic area without creating interference with each other.
Retail stores? Sure, what you are saying makes sense.
You won't be able to disprove the legitimacy of the concept of rights with a trite example, it would require a more serious discussion than we'll have here.