1) An ex-post-facto law is one that punishes you for what you did before it was passed.
A confiscatory ban is NOT an ex-post-facto law, since it says 'AFTER today, no more'. An example of an ex-post-facto law would be 'If you ever in your life posessed an AR-15, you are guilty of a felony, 10yrs'...
2) Use it for something where it doesn't matter, or sell it on Gunbroker...