I disagree, it is a MASSIVE pain in the butt to keep track of all online purchases through the year. Geez I must buy from about 30 or more different websites through the year, some take out taxes some do not. Tracking that would take much of the convenience out of online shopping. If we are going to be taxed, take it out automatically, don't saddle the consumer with it.
Bingo. If the tax is going to be collected by force, then it will be an expense whether the enforcement is on the business side or the consumer side. The expense to collect on the business side will be cheaper than the cumulative expense amongst all consumers if done on the consumer side.