Neither can keep up it. Enforcement is on the IRS, tabulation is on Reverb(s) of the world. It's absurd to put the threshold so low.
$600 comes from the general income reporting requirement. Any income earned $600 or more requires 1099 to be issued.
They've decided to consider even private sellers on these sites as self employed people and hence the general 1099 requirement is applied.
I don't think it's right. If somebody is earning significant part of their income buying and selling on these sites, then probably they should pay income tax on the profit. But if it's just a hobbyist selling something once in a blue moon and it's clearly not professional and significant, then it's not right.
IMO there should be two fold annual threshold. If x% of you annual income comes from these sales and if it's more than y$ in a year, if either threshold is met, then you have to pay income tax. If either falls below, then not.