Here's a clip from an article in Linn's Stamp News about the new eBay shipping and it's tracking:

"For this service, tracking is not available via the Postal Service’s tracking program. It is only reachable via eBay’s purchase history and thus is restricted to the buyers and sellers of the item."

If you want to read that article, here's a link to it:
https://www.linns.com/news/us-stamps-postal-history/stamps-category-on-ebay-to-see-new-postage-option-with-tracking#.YCWSmHHO0gY.twitter

For information on problems reported by eBay sellers, here's a link to an article on Ecommercebytes
https://www.ecommercebytes.com/2021/02/04/sellers-report-issues-with-brand-new-ebay-shipping-option/