Amazon pays applicable state sales taxes on the things it sells as its own products. It doesn't collect and pay taxes for third-party sellers which are the majority of its business. But that's not an unfair advantage. If you set up a physical farmer's market or neighborhood garage sale where you provide a platform to third-party vendors, their tax compliance is their business. The state doesn't make you collect and pay their taxes. And of course in practice, next to nobody forks over 6.375% of the cash from their garage sale or fresh-veggie booth to the state revenue agency.
If I introduce you to John and then you sell John your car... it's not my job to collect and pay the relevant taxes owed. Nor if I'm a newspaper publisher who sells you a classified ad that results in a taxable transaction. Nor if I own a farmer's market and rent you a booth to sell your free-range organic kale. It's more streamlined and convenient and has all the other benefits that have made them a success (including their shipping services), but at the root of it that's all Amazon is doing in these transactions. Connecting buyers and sellers while being themselves neither. Which isn't typically something that comes with tax-collection obligations, regardless of whether or not it's done online.