How to report profit on 1099-K and are receipts needed?

I've been interested in selling some items on services like eBay. The $600 limit has been a barrier to entry because I'm not planning to do this often. Just want to get rid of some items I don't want anymore.

 

My question has two parts:

  1. How do we report profit on the 1099-K? I see a section for reporting the total revenue but I don't see one for reporting personal expenses.
  2. Do we need to have receipts for every item for how much we spent on it? I don't have receipts for everything I want to sell. Is it okay to just put the MSRP?