Merchants with B2B orders were previously not eligible for Shopify Markets Pro.
We're releasing a change which now allows merchants with B2B orders to apply for and use Markets Pro by automating the detection of a business customer on an order. When this happens, the order is automatically switched to use the merchant as the merchant of record; Markets Pro cannot be the merchant of record on B2B orders.
You can read more about the considerations for using Shopify Markets Pro in the Shopify Help Center.