I believe Mr PotatoHead (handlers) is mulling an executive order on this, although taxes are implemented by Congress, not sure if either side would get 3/5th’s support.
What may be in the grand scheme is fees and fines upon those who have to register firearms. That would be a somewhat equivalent to how The obamaCare was funded, penalize those that didn’t subscribe to the mandate.
Can they impose fees, levies, taxes on previously purchases, probably not…that’s why a ban would be more effective for their agenda as they continue to redirect attention from the more important issue with gun violence. Their registry plan is not just for optics of doing something.