I would think that most full-time eBay sellers are generating at least $20,000 per year. That is not very much.
I have met people who are selling much more than that on eBay and they laugh when you ask them about tax reporting. Those are the people that the IRS will be going after. I suspect that this whole thing started with the IRS investigation of eBay sellers.
In the USA, you are innocent until proven guilty--except for when you are dealing with the IRS or Customs. Both of those departments will nail you to the wall for violations.
The $20,000 is gross, not net. You could be earning $1k/yr and still get hit by these rules.
Of course it is gross earnings.
Why you had best keep good records of the cost to you of what you sell.