[aidenn0]

The answer to that is "it's complicated"

In California, you were, in theory, required to pay a "use tax" on any goods purchased from out-of state, that just happened to be equal to sales tax. The reality is that most consumers didn't do this, since the only way to get caught would be if they audited your return.

On top of that, if you maintained a commercial presence in the state you are shipping to, sales tax was required. So, any national retailer that also sells online had to charge sales tax.

Amazon was in a gray area where they had shipping warehouses, but not commercial offices in California. They ended up making some deal a couple of years ago where they would charge sales tax online. I'm not sure what Amazon got out of it though.

So you used to be able to illegally get away with buying online without sales tax in some circumstances, but it is no longer the case for Amazon.