As ferncliff-39 said, if someone else has access to your social media account -- if they know your account password, in other words -- and that account is linked to your eBay account, that user can get into your eBay account through your social media account.
They cannot get into your eBay account simply by knowing your username or email address; they must be able to get into your social media account itself.
If someone else knows your social media account password, either change your social media account password, or unlink that social media account from your eBay account to keep that person out of your eBay account.
Then examine your eBay account closely to make sure that no contact information has been changed. You may also want to change your security question in case it was viewed by that user.
