I'm legally obligated to collect sales tax from buyers if I deliver the purchase to an address within the state of Iowa. If I deliver out of state, no sales tax is collected. If a buyer is from Iowa and provides their dealer sales tax number, no sales tax is collected. I expect that is the same with the sellers you are complaining about. If they deliver to their own state, they need to collect it. If they deliver out of state, probably not.

If you attend a live auction in your state, do you refuse to pay sales tax to the auction company?