I have sold a variety of items online this year. I would not really say I have a real business more like a yard sale where I do sometimes buy things to resell (but it's rare). It's usually things I have lying around the house, gifts, etc. I have filed a schedule C in the past for things like this, but I did it under the incorrect assumption that if I had received the item as a gift or it was purchased for me that the initial cost would be $0 and the amount I sold it for would only be profit. I now know that the initial cost basis should be the fair market value of the item on the date it was received.

My question is how do I estimate this initial market value? I have the receipts for some of them so that's easy. However, I don't know what to do with other items I purchased a while ago (and don't have a receipt), things I received free or as a gift, and things that were parts of other things I own (for example: selling a car part that was part of my car and that I had not purchased separately). If I do some research and come up with a reasonable justification is that enough or is there a specific process for doing this? I know there is a way to do this if I wanted to figure out the current market value if I was donating an item, but I'm trying to figure out what the market value was when I received the item. If I don't have any proof, am I just forced to assume a $0 cost basis?

I think it's still going to come out to either $0 profit or even a loss (which I cannot actually claim), but it's my understanding I should still file a Schedule C anyway.