The thing is, I'm suspicious of the intention behind changing the reporting threshold from $20,000 to $600 for third party sellers(PayPal, etc.) because it seemed to coincide with the push to hire 87,000 new IRS staff. They are not going to get that much more dollars out of the wealthy, no matter how many more staff they have, so what are all these employees going to be doing?
And they know full well that most people who engage in casual sales do not have receipts for items they may have bought years ago. Yes, you could include the sale of such items on your return and list the price for which you bought it and the price for which you sold the item, but then you'd be relying on the IRS to take your word for it. I recently sold some Legacy Audio speakers that I bought in the 1990's and I could only guesstimate what I paid for them. I do know that I sold them for less than what I originally paid, so there would be no tax liability, anyway.