Not sure how much it's used on ebay.com but from this presentation it seems to imply that it's used extensively on the site and other projects at the company.
Yes, that is correct. The presenter in that video is Patrick Steele-Idem, the original creator of Marko. eBay.com is predominately built using Marko and Lasso https://github.com/lasso-js/lasso
https://www.youtube.com/watch?v=i6eZA8Y_GgA
(their front end stack is presented at about 4:00)
BTW it's a great introduction to Marko compared to React and Vue.