How to handle items I sold in a Facebook group

I think I know the answer to this, but wanted to check to make sure I'm doing it the right way. Last year in the fall and before Christmas I sold some of my kids clothes and toys in a local Facebook sales group that my kids weren't really using. I am confident that the sale price of all items was less than the value paid. Some of the items were gifts from relatives, but I sold everything at such a low price that I'm sure it wouldn't have been more than what they paid for the item. 

 

I'm putting these sales in the same category as garage sale proceeds, and from what I understand, the money I received would not be counted as income because they were sold for less than the original price. But do I have to have receipts to prove this? I would think that it would be logistically impossible to do so. Am I correct in not considering these sales as income?