Even if you've sold a stock you get one, because they count any stock sold as pure profit, even when they know you've bought a stock that offsets it.
But they don't know how much you paid for the stock.
All of these things track income, the IRS doesn't track your expenses -- you do. You are motivated to properly track your expenses so that you can pay the proper tax (if any), rather than the one side view that the IRS has (i.e. the income statements).
I don't have an answer for someone selling off a bunch of stuff on eBay. But, in the end, it IS income. In theory, if you have a garage sale you're supposed to report all of that as income. Obviously, most people don't.
Why eBay sent the forms out, I can't say. Was it activity, was it simply raw volume? At one point the IRS considered having EVERYONE send 1099s for EVERYTHING. It was an insane requirement.
But here's a system that Just So Happens to offer accurate accounting of income, and they do it "for free". It's also representing a large enough nut that the IRS has deigned for eBay to report it.
Are they actually going to do anything about it? In most cases, unlikely.
But did you sell $20,000 worth of "collectables" or "vintage camera gear" or whatever? That's enough to be worth putting an investigator on the case to get their cut.
Arguably, today, $600 is too low. $600 has been the reporting limit forever. At least 20 years. It should be bumped up to take inflation in to account (especially today). But that will likely take an Act of Congress to get changed.
Undoubtedly, They(tm) are looking for patterns. If you "clean out your house" each year to the net of several $1000's of dollars in sales, then, perhaps, you're cleaning other folks houses out as well.
But in the end, talk to a tax attorney. The IRS auditing folks are pretty backed up so I've heard, but no reason to walk in to their crosshairs if you can avoid it.