Pretty trivial to calculate all taxes given the zip code.
Along with some states charge taxes differently for food products or pharmaceutical products or charge no taxes on some. If you are going to re-sell an item you have a state tax waiver from the state and must pay the tax at final sale to consumer. Quite a headache for an ebay shop having to track state/county tax levels, waivers for resellers, not all items taxable in some areas. Should drive folks back to stores. OR; a niche for a new business model for an intermediary store in a non sales tax state that 'purchases' the item for you for a fee smaller than a tax and it drop ships to you.