The bottom line is that when you make a purchase, you are supposed to pay the sales/use tax. When buying on Ebay over the years, while we were not having it added onto purchases, if your state has a sales/use tax, we were supposed to pay it on our state tax returns. Obviously, most people did not do so. Now, the issue is being forced. If you live in a state that does not have a sales tax, you won't be paying one. I know there is a lot more to it as far as how it all worked in the past. So, when companies are charging the tax, it is overall, just what you would do if you went to your local shop and purchased.
I remember 30+ years ago, people driving hours to make large purchases in order to avoid sales tax. For a long while, the internet made that even easier. The states realized the revenue being lost and rectified it. I don't like it, but I get it.