How would you say 'he sold fish' in the sense that he did that continuously for a period of time, e.g. 'Last summer he sold fish'?
That's exactly "sprzedawał".
Unfortunately, the perfective/imperfective difference doesn't translate easily into English. But I don't know if with no further context like the one you provided, the English sentence could be understood in this continous way... could it?
Darn lack of perfective/imperfective distinction in English :| Makes everything more complicated, because then often "well... one version is a lot more probable, but the other one is technically correct as well". It does make learning harder... but what can one do. Added your answer.
When a man says, "I remember that I was selling fish," how is it in Polish? Is "sprzedawał" also suitable for "I" and "you" (sg.m.) in Past Imperfect?
No, that would work in Russian (maybe you know some), but not in Polish. In Polish, every grammatical person has their own form, and it also accounts for gender of the subject of the sentence.
So, a man will say "Pamiętam, że sprzedawałem ryby" and a woman will say "Pamiętam, że sprzedawałam ryby".
Yes, Russian is my native language :). Unfortunately, Polish lessons in Duolingo are not accompanied by grammatical explanations. I'll try to find a good textbook.
Thank you very much for your answer!