When you create a new Item, you can assign a sales tax on the selling side. On the PO side there should be an option for no sales tax. I'm a reseller so when I convert a Sales Order it automatically maps over the sales tax and then I have to manually remove them. As a reseller, I don't pay sales tax on items I purchase for resale. It's an easy fix inside of Zoho Books in the Items Module. Provide the option as you do for the Selling side but under the PO side.