Subject came up at work when we were talking about cryptocurrency. Garage sale money (or ebay/online sales) = NOT INCOME unless you’re selling an item that has improved in value. Anything that has appreciated is subject to capital gains tax on the profit.
It’s our understanding the laws changed recently so money sharing apps like Venmo/Cash App/Paypal will be sending 1099-K when you have transactions exceeding $600. So if you downsize your house and sell off things on ebay, you’ll have to be able to prove what was or was not profitable (which would require knowing the cost basis of your items).