I have a hunch that this will, in the end, be a massive win for large retailers vs. small ones. The task of figuring out how to calculate tax for all states is more or less the same amount of work regardless of size, which means for someone like Amazon it's more or less trivial, but for a mom-and-pop store it's a major hassle.
Many online payment gateways already offer some kind of sales tax API. Keep in mind in California, sales tax changes per county. In places like Tennessee, the tax rate can change if your item is a grocery or isn't a grocery. There are companies that already sell massive lookup tables or API access to calculate a lot of this stuff. I think for most retailers, this won't be that big a change. Most mom-and-pop online st…
However, this is a HUGE change. Instead of registering for a sales tax permit and remitting tax for one state, smaller merchants will have to do this in many states if they meet a certain threshold. They will likely have to use sales tax compliance software to aggregate all of their transaction data across multiple platforms (Amazon, eBay, Walmart, etc) and their shopping cart, then report / remit accordingly.