Ask HN: Costs for US sales tax compliance for a two-sided marketplace

I'm consulting for a US-only startup that has approximately 2000 sellers. We don't yet facilitate payments on site because we are worried about not being sales tax compliant. But now we're thinking about biting the bullet.

Assuming we sell into all states and that our transaction volume will reach $10 million USD, what would be the cost of calculating, collecting, and remitting sales tax to the relevant states, counties, and cities?

Assume too that we only sell one type of product (i.e. we are a specialist marketplace not a general one).

It might be worth separating into setup and recurring costs.

Any surprises in store? Or recommendations in terms of tooling or vendors?