Most states are requiring taxes be paid for internet sales whether in state or out of state. Since most of the audio related businesses are small companies, they often outsource the tax collection and reporting to companies like Taxjar....but there are many others and most sellers discuss their tax policy somewhere in their "fine print" pages.
As for fees, Audiogon lists all of its fees upfront but you have go go to he help page to get the link to the fees page.
https://support.audiogon.com/hc/en-us

