If whoever bought it knew how to fix the guns, it would be dangerous.

If the guns *can* be fixed without major work (usually replacing the "receiver" or something equally difficult) then they aren't "non-functional" and you have to go thru the background check and pay the transfer tax.

Besides, you don't need the guns working to be dangerous in a tank. Those things can run over cars and right through buildings.

No more so than a bulldozer. Or a large truck.