Re: How about fixing the existing tax code to cover this??
VAT and sales taxes are taxes on consumption - not on business turnover! This is really important to understand. It's basic tax incidence - because the tax falls on the consumer, not the business.
The reason that it's not a turnover tax is that businesses don't pay VAT. Therefore if you're a company that sells to both consumers and businesses - only the consumers pay VAT - i.e. they end up paying more for the same goods than the business customers. It's not a turnover tax, because you ignore it in your accounts - I mean there's lots of shuffling around, but sales and costs are reported excluding VAT - all that stuff is diverted into your VAT account and then the net of that paid to the government every quarter - or month if you're big enough. So there's all this money changing hands, and the finance department have to worry about cashflow (as you pay invoices with VAT) but all the business people only talk net figures, and ignore it.
I don't believe Tobin was calling for a tax on retail! The Tobin tax was designed to make short term share/bond ownership less profitable - and therefore relatively rewared long-term ownership. It was not designed to punish retailers for choosing to make smaller profits than their rivals.
It's also not a turnover tax either. It's a tax on buying shares/bonds. I've not read his papers, but I'd assume it's there as a "sin tax" i.e. to discourage certain behaviour. As well as to raise revenue supposedly painlessly. The turnover of a finance company is the services they're selling to their customers - whereas a Tobin tax would be a tax on transactions - but nobody but market makers make all their money out of transactions, so it's an increase on costs. A turnover tax on financial companies would be a tax on their turnover - i.e. a percentage charge on their total sales.
As happens HFT is a great idea, in theory. It makes markets more liquid, more transparent and lowers transaction costs and arbitrage. The reason it's probably not a good idea is the risk when that market liquidity is withdrawn - which is why traditional market makers are probably a much better idea. But wasn't Tobin's work from before HFT was a thing?
Anyway when the Swedish tried a Tobin tax something like 70% of their stock market transactions disappeared overnight. Perhaps they set it too high? However London has had a stamp duty forever, and is the top or second financial market in the world, so it's not impossible. But when the EU looked at a Tobin tax a few years ago, the Commission's calculation was that it would raise something like €20-€30bn - which isn't chicken feed but isn't that much over such a large market. However it would reduce growth in those countries by 0.1-0.2% - every year. Which meant I think that it wouldn't even have a positive effect on governent revenues in the first year, basically other taxes would fall more due to the drop in growth. As well as basically costing a growing amount of money to the economy forever. Also if they made people more reluctant to buy government bonds, it would have made the Eurozone debt crisis even worse.
Anyway I'd argue the most advocates of a Tobin tax are engaging in wishful thinking. The idea that you can raise taxes without pain is just silly. You can't make someone else pay them, as that someone else will then have less money to pay you, and so everyone suffers.
But you've also not understood it. It's a financial transaction tax and not a turnover tax.