Yea, this isn't an eBay thing. It's a Facebook thing. Facebook's requirement for being used to sign in to the eBay app. Being you allow tracking.
That's really why these companies want you to use them for sign ins on third party services. It's not to make your life easier by having less logins to manage. It's for them to dig deeper into your online activity, interests and purchases.
Setup individual accounts for each website. To reduce cross account tracking.