First of all why does no one have an issue with paying a tax on something that you bought with income you were already taxed on. Stop taxing the same money over and over. Also we already have capital gains taxes so if anything you trade has increased in value since you bought it, it would presumably be taxed yet again. Secondly this would probably bring its greatest burden on people with retirement portfolios which constantly move money to ensure they gain value. Just remember everyone, taxation is theft!