McCain's campaign didn't return repeated calls and e-mails to confirm where the 50,000 number came from.
But it clearly didn't come from eBay.
"We're not exactly sure where he got the number. We don't even calculate according to the U.S.," said eBay spokeswoman Deanna Monestero. "I don't want to say it's wrong, but I can't trace it."
The latest figures for the U.S. date back to 2005, when an A.C. Nielsen study revealed 724,000 Americans used eBay to make some money. But the survey didn't break down the number to figure out how many were just making a few bucks compared with the number making a living on eBay.
Back in 2003, a similar survey found that 430,000 Americans made all or some of their income from the internet auction site. But once again, the survey didn't break down the "all" and "some" income numbers.
That same year, Jim Griffith, who is eBay's so-called "dean of education" (who knew there was one?), was quoted in a Colorado newspaper as saying, "More than 20,000 people earn their living selling on eBay in the United States."
But a company spokeswoman couldn't say where that number came from, either.
These days, the company only keeps track of eBay entrepreneurs worldwide. That number, based on a 2006 Nielsen study, is estimated at 1.3-million. But again, it doesn't distinguish between full-time and part-time on-line auctioneers.
The bottom line: Lots of people make money on eBay, but how many depend on it to pay their bills remains unclear.