"They sell wine in his bar."
Translation:One sprzedają wino w jego barze.
Polish only has "true" possessive pronouns for the first and second person. All of them are inflected for case. However, if you want to express possession for a third person pronoun, you have to use the genitive form of the regular personal pronouns (jego = of him, jej = of her, ich = of them). Since those are already inflected forms, they can't take yet another case, so they remain unchanged, no matter what.
I'm still largely imitating what I see for word order rather than grasping it intuitively, so I put the ‚W’ at the beginning here, like „W jego barze oni sprzdają wino.” Duo didn't like that though. I'm guessing that it puts the emphasis in the wrong place? Or is my error something else?
Yeah, I guess it's a matter of emphasis only, your version sounds to me as if THEY is the most important part, that it's them who sells and not him (the owner) himself. Without a context, I'd read the original question as a simple "what do they do for a living", or alternatively "where do they sell wine".
Oh, also you 'ate' one letter in sprzedają, but that would probably still be accepted if the whole sentence is in the course creators' database.