I have no idea what schedule C or D mean. I am checking as Self Employed and use 1099-K on Turbotax. I don't recall Turbotax asking me about any Schedules. But to clarify your assumption, like the original post says there is a MIX of personal items and various items that I buy to resell. I am given two options at Turbotax to calculate cost of goods sold - Inventory or Supplies. I don't really have active inventory, I just sell randomly whatever I have at the moment. I may have 10 items for sale one week, and 100 next week, so it varies, and items I sell vary as well, there is no constant inventory.
Schedule C reports activity from Self-Employment. As you enter information under the self-employment topic in TurboTax, the program prepares Schedule C as part of your tax return.
You are not required to report inventory (or cost of goods sold) if your gross receipts (income) are less than $1 million. In this case, simply report the purchase of sales items as Materials or Supplies (your choice).
You can either report all your purchases as "merchandise for sale" in one lump sum or you can separate the items by category under Supplies. For example: Personal vs Resale, or Furniture vs Kitchen Appliances, Clothing vs Books. Again, this is for your information only. How you report your general expenses is not important. Just be sure you report an expense only once.