We own a hardware & lumber supply store. I am a little confused. When we purchase lumber and hardware to resell what category to I list that expense under?

Items for resale are in general, inventory, and when you purchase these items they become part of your current inventory (asset) and when you sell them your expense is cost of goods sold (COGS)

Here is an article to review https://quickbooks.intuit.com/learn-support/en-us/manage-inventory/understand-inventory-assets-and-c...

Some items you purchase may be difficult to track specific quantities sold, e.g. nails by the pound, etc due to nominal losses or to even account for damaged goods so a quarterly or twice annual inventory count and adjustment may be necessary