The defects in the process:
(1) If the law applies only to businesses that do, say, $20,000 in business, they miss out if they only start collecting it when a seller hits $20,001.
(2) State laws generally require that any overages collected by a vendor go to the state, and not back to the customer at a later date

Keep in mind that Ebay is not charging the sellers - they are charging the buyers through the sellers. They are not taking sales taxes out the seller's "share"of