We desperately need a tax on securities. I don't particularly care about taxing luxury products, because those products are produced by workers, who pay taxes and buy things. But securities are the "means of production".
Don't tax the dollar value of the security. Don't force them to liquidate their shares before they can pay a tax. Just go ahead and transfer a percentage of their shares of each security to the IRS, who can liquidate it on the open market, slowly over time.