Jordan said in his tweet that he notified about the vulnerability to eBay. A screenshot published in his twitter account shows that he is able to upload a 'shell.php' file in the following location:
"https://dsl.ebay.com/wp-includes/Text/Diff/Engine/shell.php"At the time of writing, the file is still there. The last modified date of the file is December 2012. It is quite possible to modify the TimeStamp. So, we are not sure from when the file is there.
Trying to access the shell ends up in blank page. It means either the researcher have modified the shell to run only when a particular input is passed or it is not a shell.
Jordan have also discovered a cross site scripting vulnerability in the eBay Research Labs page(labs.ebay.com).