I am setting up quickbooks for a new business. I am trying to categorize entries from my bank account. I purchase an item from a vendor and plan to resell it. COGS?

What you plan on keeping on hand to sell is by definition inventory.

After you create the inventory item (plus and higher) and stock it with qty and value (comes from a purchase).  Then you sell it.  When you sell it, the cost is removed from inventory and qty goes down, that cost posts to COGS.

There are basic intro courses on the net, use them to learn accounting now before your business expands