Faire, in particular, would not have as much need for income tax integration as for sales tax management/integration as other marketplaces have had to do -- Etsy, Amazon, Wayfair, et al. Even Upwork (major marketplace for service providers) AFAIK stays far from income tax issues and simply provides the registered users with tax forms on the site for the use of the users in paying their taxes as independent contractors/businesses.

For small businesses that do sell in multiple states, there are significant USA sales tax issues that are becoming more complicated -- in addition to TaxJar there's TaxCloud which does a good job helping small businesses with multi-state tax filings. This particular one is a public policy issue that the states won't let move to the national level easily. About half the states are bought into a system of reporting and requiring taxes to be collected from sales in each other's jurisdictions. The last I checked there were something like 900 different sales tax rates nationwide since every little town can charge its residents a unique add-on to the state taxes. As someone who sells items online across the USA and world-wide, I have found TaxCloud to be an effective way of dealing with filing sales tax in multiple states. They do the filings for the customers and as long as most sales are done online it is painless. Integration of TaxCloud with existing software is tricky, I had to have a programmer pull it together but it was much easier than keeping up with hundreds of sales tax rates in constantly changing look-up tables and paying each state myself.

I am curious about the specific problem that Faire is struggling with. Perhaps some of the new state laws (e.g. CA vs the gig economy...) are driving things here in terms of employee vs contractor (income tax) though Faire is not a marketplace for services so much as products. More insights?

What kind of tax are they wrestling with? Income tax for their vendors? Sales tax? Payroll taxes? Import/export?

Probably not what you're looking for, but Stripe does have Stripe Tax (https://www.cnbc.com/2021/06/10/stripe-new-software-helps-companies-calculate-sales-tax.html).

I think “programmable payroll” is really interesting.