It's probably true that less trades would occur, and HFT profits would probably be reduced by this, but they would be able to mostly avoid paying the tax.
What follows is full of holes but is a mostly correct explanation of what I was talking about before.
Each venue maintains an "order book" for each security that trades on the venue. The order book consists of a number of standing orders, either orders to buy at $k or orders to sell at $j (k<j, with many distinct values of j and k). These orders which sit on the book because they do not have a counterparty are called "add liquidity." When you want to just execute the trade and you don't mind the spread, you place an order that crosses the spread and transact with the person who has the best standing order (selling for the least money or buying for the most money) of all the standing orders. This is called "take liquidity." My impression, which may not be correct, but which forms the basis for my previous response, is that the HFT industry primarily plays the add side right now and that most firms only take liquidity to close out positions when they are unable to close them out by adding liquidity.
What difference does that make? I'm really not following you. When they sell, or buy, they pay the tax. It makes no difference if they place a limit order or a market order.
The spread is just the difference in prices, it's not an extra fee that some pay and some don't. The only reason it even exists is that where the prices meet trades happen, so the prices never actually meet for long.
The tax becomes part of the spread. The spread is paid (in aggregate) by the participants who take liquidity to participants who add liquidity.
It's not that market-makers and HFTs would not be assessed the tax at all, it is that they would be paid more than enough to cover the tax by the people who cross the spread.
What follows is full of holes but is a mostly correct explanation of what I was talking about before.
Each venue maintains an "order book" for each security that trades on the venue. The order book consists of a number of standing orders, either orders to buy at $k or orders to sell at $j (k<j, with many distinct values of j and k). These orders which sit on the book because they do not have a counterparty are called "add liquidity." When you want to just execute the trade and you don't mind the spread, you place an order that crosses the spread and transact with the person who has the best standing order (selling for the least money or buying for the most money) of all the standing orders. This is called "take liquidity." My impression, which may not be correct, but which forms the basis for my previous response, is that the HFT industry primarily plays the add side right now and that most firms only take liquidity to close out positions when they are unable to close them out by adding liquidity.