What about taxing delivery services instead? If states charged a flat tax on UPS and Fedex, distributed proportionally to cities, would it be enough to fill the gap in state and city budgets?
Do you go after 300 million people or do you go after 10s of thousands?
This is a problem with your states, not with the general idea that in the US the internet retailers are getting away with helping people avoid sales tax at the moment.
This was inevitable, if you didn't see it coming you're a fool.
Also this type of 'problem' is easily automated by computers, you're just going to have to pay for it.