If you got 500$ - 100$ in tax for the card and it drops back to 50$, you can just buy it back with 400$ you still have left...

They're talking about a wealth tax where you have to pay $100 even though you never sold the card. (But it's double bullshit because something as small as $500 is never going to face a wealth tax, and $100 (20%) is a way higher tax rate than anybody would pay.