I am a reseller, I sell both items I have had for years in my closet, and Items I purchased to resell. How do I separate these on my taxes & sales totals?

I resell on Poshmark, Ebay, Mercari, etc., and keep track of my sales. However, I sell a lot of items that have just been sitting in my closet, which I sell for less than what I purchased them for. I also sell items that I purchased at thrift stores, and resell them for profit. How do I go about tracking these items, and calculating sales? The sites show my total sales, but not all of them were profit. Please help!