@Tor, you misunderstand how default tax codes are applied in Manager.
A tax code set for an inventory item is applied to it whenever it is invoked for a transaction, no matter what type. For a sales invoice, the tax calculated is credited to the Tax payable account as part of balancing the amount of the inventory plus tax debited to Accounts receivable. (The remainder is credited to whatever income account is selected.) Thus, the tax code selected for an inventory item has nothing to do with the income account where the value of the item sold is recorded. For a purchase invoice, the tax on an inventory item is debited to Tax payable, where it offsets the tax collected when the item is sold. In this way, Tax payable records the net of input and output tax, resulting in the figure that you owe the tax authority.
A tax code set for an income account applies to amounts posted to that account. The amount credited to an income account for sale of an inventory item has already been taxed at the item level. So no tax should be applied by default to an inventory income account, because that results in double taxation. Default tax codes for income accounts should only be chosen for accounts where you post otherwise untaxed sales, such as of services. (This is partly why sales of inventory are recorded by default to Inventory - sales, rather than being lumped in with other types of sales.)
@Patch is correct. The same tax code should always be used for purchases and sales of the same inventory item. This way, you get the benefit of offsetting described above, as well as the ability to use all of Manager’s built-in reports and automatically created accounts. This concept is so deeply embedded in Manager’s design that, if you ignore it, you cannot use any of Managers built-in tax accounts or reports. You must bypass the Tax payable account entirely. Procedures for doing that (for a completely different reason of multi-component custom tax codes) are described in this Guide: https://www.manager.io/guides/7171. It would be a lot of work for, in your case, no return and many possibilities for error. You would be far better off using the standard Icelandic localizations and relying on the automatically created accounts related to inventory and taxes.
As a side note, there are many Manager users in Iceland. As far as I know, none of them do things the way you are trying to. Certainly no one has complained about using the Manager structure as designed in the past five years.