My idea was slightly different: there is also a transaction tax for all transactions except for "investment accounts". I view these accounts as accounts where the owner sends the money and may invest as much as she wants in it (buy stocks, sell them, ...). The tax is paid when funds are withdrawed from the investment account to a normal account (you cannot buy anything else than financial products in investment accounts).
So, the transaction tax should be paid only once by the "buyer" (the person that sends the money), not by the "seller" (the person that receives the money).
Of course, the amount of money collected by this tax would be much lower than with AMT, and the tax rate should be higher. But even though it were of the order of 5%, it would be much lower than current sales tax.
Additionally, my interest in this kind of tax is not so much to "penalize" rich people that invest in stocks, but penalize "black money". In my country, black money appears everywhere, and "bad" people do not pay taxes, only "good" people pay them. Having a transaction tax (and abolishing cash, of course; I still have not seen it in the posts I have read, but for me this is essential) would make bad people to pay taxes, too.
