eBay doesn't. YOUR state government does. eBay just collects it acccording to the law and regulations of the applicable state (the one the item is being shipped into).
Most states with sales tax include the s/h fee in the "price" to calculate the tax, and many of those who generally don't include it do include it in certain circumstances which tend to include the circumstances of most eBay transactions (e.g. when "handling" or other charges can be included by the seller, as on eBay where it is shipping AND handling).
