So Amazon is more than double but maybe not 10x? Amazon GMV is supposedly in the $250B range but they don't report it. And it's obviously hard to compare such different business models.
So you're comparing Amazon sales to eBay market value (total stock price). Yeah, it's completely meaningless comparison [Edit: I was wrong, it's Amazon sales versus eBay's gross merchandise value but it's still entirely off as a comparison].
Btw, my quickly googled references show eBay's net sales as 10.75B and Amazon's at $141.92B, which is indeed a more than 10x relations (when I think all other statistics show Amazon an order of magnitude ahead, not that I initially made any strong assertions about this but anyone looking has notice Ebay's star fading).
[1]https://amigobulls.com/stocks/EBAY/income-statement/annual
[2] https://www.digitalcommerce360.com/article/amazon-sales/
https://en.wikipedia.org/wiki/Gross_merchandise_volume
eBay states there's here: https://ebayadvertising.com
The question is what is the GMV for Amazon's entire marketplace vs. their first-party Sales.