There's plenty of smaller sellers that would love to be in compliance, but due to the nature of US sales tax and how FBA works (i.e., you don't know where all your product has been stored) it's practically impossible for them to be in full compliance, so they don't even try. Sometimes better to stay off another state's radar completely than get drug into a spat over partial compliance.
Say you bring a new widget to market, as a small business, and you start selling via FBA. Pick your favorite kickstarter project and imagine it's you.
There are thousands of sales tax jurisdictions in the US, between states, counties, and cities. There is no way you, as a little guy, can manage all that.
It's not though. If I live in California and have a presence only in California and ship a widget to Pennsylvania, I'm under no requirement, as a matter of constitutional law, to collect PA state sales tax on that purchase. What the buyer states, on their own state tax forms, is their own business.
This specific case, about sellers affiliated with Amazon but not part of Amazon, is ultimately for Amazon, its affiliates, and the states to resolve. There isn't a role for an independent startup. The same would apply to present or future networks of sellers in the same vein.
Your response is why my other related comments mention only through legislation this gets fixed. Everyone is complacent because no single entity is held responsible, and people are evading taxes because of it.
Attempting to justify tax evasion with constitutional law is unacceptable.
Right, and how are you proposing to enforce that legislation? Passing laws solves nothing by itself. How would you get buyers to file whatever you ask them?