I would imagine, as immie said, anything used and being resold is not taxable, because the tax was already paid previously by someone.
As far as thousands of new items being sold on places on the net, they will have to set up their system similar to buying from any retailer that operates in every state....like Sears or jcpenney....off the net purchases are charged the sales tax for the state the person is buying it, is from....it's a simple computer program.
and yes, I agree they have a spending problem and States should cut their own spending as well....
BY LAW anything that you purchase from another State, tax free, you ARE SUPPOSE to claim this on your Income taxes for your state and pay your State sales taxes on it.
Most people IGNORE the law on this.....and they get away with not paying their state the sales taxes on their purchases owed to the State.
As far as, what power does the federal government have to enforce State laws.....
this, I don't know....I would have to do further reading on it....