Oh I'm not saying I won't pay the online sales tax on physical stuff, but if they start doing so for digital transactions there might be a problem.
It can still be done, provided that the proper recording and payment systems were in place.
Let's say you download MP3s or e-books or something from Amazon (I'm just using them as an example here). Now, Amazon is based in WA and has distribution centers in KY, PA, AZ, and a couple other states. Now let's say that you, the purchaser, are living in a state where the retailer has no physical presence--let's say Maryland or something.
So you put in your credit card info, etc. and when you do, you will have to provide a physical mailing address. (I don't know if they make you do that now for digital downloads, but they could.) They figure out that you are purchasing this stuff from MD, and charge you the appropriate sales/use tax. You pay the purchase price (plus tax). What Amazon could do is take the amount of tax you remitted and put it in some sort of escrow account to be paid over to the MD state revenue commission at a specified time, using a database to keep track of it all.
It's not impossible, but the record-keeping system would impose greater costs on online retailers, as well as forcing sales prices up. Now, large e-commerce vendors such as Amazon, Overstock.com, etc. would easily be able to absorb such costs (which of course, would ultimately be passed on to customers). Smaller merchants, however, could be forced out of the market if the cost burdens of following such new regulations increase their overhead too much.
(Note: Think about the fact that big corporations don't always oppose regulations and taxes--they will support them if it hurts their competitors more than it hurts them, and/or increases their market dominance.)TL;DR--Like I said, I'm not happy about it, and neither are the big online retailers. But it can be done.
