Where are the notifications actually from?
https://pages.ebay.com/securitycenter/stay_safe.html#fake_emails
If the notifications are not actually from eBay, they may be a mistake or a fraudulent email designed to get you to click on a link and give up your credentials.
Do the transactions appear in your Purchase History?
If transactions appear there that you did not make, see here:
https://www.ebay.com/help/account/protecting-account/get-help-hacked-account?id=4196
