With the stroke of a pen you will be a criminal

[Double Naught Spy]

Congress cannot pass a law that makes a law abiding citizen a criminal according to the constitution.

I believe that the ex post facto ("after the fact") laws pertain to prosecution for an act committed before a law end into effect. You you could not be prosecuted for buying a bumpfire stock when it was legal to do so, but that doesn't mean it will remain legal for you to own one after the law went into effect. Similarly, while sellers may have them for sale before such a law would go into effect does not mean they can keep selling them after it goes into effect.