I've been an eBay and Amazon seller for almost 20 years. I've been following ongoing discussions on this issue on the Amazon selling forums for years.
What the states are trying to do is claim a seller has an economic nexus in their state. Many states so far are going with the 200 transaction or 100,000 revenue criteria to determine your nexus.
It's easy to reach the 200 transaction limit on smaller items. I sold around 3500 items on Amazon last year, and around 350 of them were to California. California is sending out threatening letters to sellers about collecting tax this year and potentially even going after back taxes to the first time you sold something to a buyer in their state. Some sellers are registering for sales tax only to then be confronted and told they have to file income taxes in the state as well since they are now a registered business. Can you imagine a small seller having to file sales and income taxes in up to 50 states a year? I hate to do it, but I'm going to scale back on transactions this year in an attempt to avoid the 200 limit. I'll eliminate sales of items that might only make me a couple dollars profit each to do so.
There are services such as TaxJar that claim to make this whole process easier on us, but they fail to notify people of the income tax possibilities and the fact that once you register for a sales tax license in the state, you're screwed. These services charge a monthly fee and a filing fee for each state you have to file in. It can easily cost thousands of dollars a year for the privilege of filing your sales tax.
Some of these state taxes people are noticing automated on Amazon and PayPal are through the marketplace facilitator tax. In those cases, Amazon or PayPal is collecting and remitting the sales tax for the seller. That's the only way this is going to work without putting an undue burden on small businesses. Sellers are begging California to implement this, but they haven't done so yet.