it may be that he is suffering from backscatter spam.
Basically, if someone spoofed his email address or sent from his account to say 1000 email addresses and 50 of those email addresses that he sent to did not exist, he would probably receive 50 bounce back emails to his inbox saying the mailbox didnt exist.
Best thing to do would be to create a rule within GMAIL that moves email with a common phrase to the delete items so you can filter out your inbox.
Other then that you should contact GMAIL and Ebay support to let them know.
Make sure you pick a secure password going forward.