Whether the money you bought something with was taxed or not has nothing to do with it.Wait? The govt wants me to report proceeds from sales of things I bought with income that was taxed? I don't think so. I lose money on this stuff, so no capital gains to report.
Again nothing has changed - any money that you receive for any reason has always supposed to be reported. All that's changed is that other parties involved in the transactions (middle men like Ebay for instance) are now more likely to report their involvement (notifying us with 1099's) at lower levels than before.
Welcome to the age of computers!
Back in the day, keeping track of and reporting these small amounts would've been too costly - now it's just an additional line or two of code.
Leaving the only way to have no involvement with any of this as doing only transactions that are one-to-one, in person sales with green money that never gets deposited anywhere.
But again - no tax law has changed - these small transactions always had tax liabilities - we just lived in a more cash on the barrel head sort of world.