You've come to the right place. Let me provide some information on the item list in QuickBooks Desktop.
You're correct! When creating or editing an inventory item, the "Purchase Information" section relates to the purchase order you created. And the "Sales Information" section relates to the invoice.
On the other hand, if you purchase an item for a resale it'll be listed as Cost of Goods Sold (COGS). The item can be an expense if it includes as cost of the sellable item but can still be usable in the future.
For example, you're selling a painting and purchase the frame from your vendor, then buy material like paints and brushes. The frame will be listed as COGS while the paint and brushes will be an expense since you can still use those items in the future.
I also encourage seeking help from your accountant so they can offer you some advice about the inventory items in your QuickBooks Desktop.