The largest burden would fall on small business's that have internet sales. To subject small online retailers to the tax collecting efforts from 50 different states and who knows how many municipalities will be a disaster.
Even walking into a store here in Pennsylvania and trying to figure out how much sales tax you owe is next to impossible because the law is so complicated and riddled with exemptions.
Most clothing is not taxed. Sewing thread is taxed. Unless you tell the clerk you are going to use it to make clothing, in which case it is not taxed (I am not making that up!) Six or more donuts are not taxed as they are assumed to be grocery, but five or less are taxed as it is assumed you will eat them on-premesis and hence they are restaurant food.
The tax rate is 6%, unless you are in Allegheny County where it is 7%. How is an online retailer half a continent away supposed to keep track of all that? Local shop owners who were born and raised here can barely do it.