We recommend that Marketplace sellers use the new MP_ITEM_SPEC version 4.2 that has removed the removed ProductTaxCode attribute. Tax codes are now auto-generated and assigned to items during the item setup process.
Sellers can override a product tax code that was auto-assigned when the item was initially set up. The MP_ITEM_MAINTENANCE spec version 4.1 has the ProductTaxCode attribute that you can use to override a tax code.
For more information and to download the Item spec versions, see Item spec v4 in the Guide.