You didn't indicate if this was an item from a store as opposed to a used item from a private seller, its implied its an item from a store. It's probably legit as most states are successfully collecting sales tax on online transactions. The service fee is BS in my opinion and depending on who actually collected it I'd argue for a refund. If the online store collected it then get a refund, if Agon did then you could have just transacted with the online store outside of Audiogon. Too many greedy hands into everyone's pocket- vote with your wallet and don't shop with vendors adding extra fees for everything. Pretty soon they'll be asking us to pay for keeping their lights on and their employees medical benefits .

There is no sales tax for items bought between states.  However, states differ on their online sales tax rules.

I find it repulsive that online retailers like eBay, etc. were all excited to become tax collectors.  

Retailers that charge sales tax and pocket the money can be prosecuted if they are caught. Retail sales are very hard to track/audit.  Most retailers will knock off the tax if you pay cash. That should tell you something. Perhaps several somethings. 

Audiogon collects the service fee!!

Audiogon Service Fee

A service fee is charged to buyers when purchasing an item from a seller; the amount is dependent on sales price. This is used to support the marketplace, develop new features, and provide support to our customers and community.

@yesiam_a_pirate" There is no sales tax for items bought between states.Do You Charge Sales Tax on Items Shipped Out of State? + FAQs"

Yes, in some cases, sales tax must be charged on items shipped out of state. In fact, 45 states (plus Washington, D.C.) now require out-of-state sellers to collect sales tax, generating over $23 billion from remote sales in 2021 alone