What makes you think someone else logged in to your account?
If eBay sent you a message about a sign-in from new device, is it possible the sign-in was actually done by you? You might get such a message after signing in using a new device or browser, or a browser that you had cleared cookies from, or if you changed your IP-address, or used a new network connection.
If you are sure that the sign-in was not you:
Does anyone else know your password, or have access to your computer or phone, have access to you email account, or any linked social media accounts?
Have you clicked on any links in any emails and provided credentials after doing that?
If you think someone has gained access to your eBay account, see here:
https://www.ebay.com/help/account/protecting-account/get-help-hacked-account?id=4196
