To make an inventory item available in your assembly build but not for sale, do not choose an income account on the item record.
By not setting an income account on an inventory item’s record, the item does not appear in the sales transaction’s items list. The item is sold as part of the finished goods assembly the item belongs to, which has its own income account.
If you do not set an expense account on a non-inventory item for resale, the item does not appear in the purchase transaction’s item list.