How would you describe the difference of the two words as a native? In English I don't see a big difference in meaning, or am I wrong here?
Well, "you were selling..." implies having succesful sales for a period of time, "you sold" at first sounds like it was a one-time thing (at least to a non-native), but unfortunately it's not that easy and it could work in this sentence as well.
More clear in Polish. "sprzedawałeś wtedy..." = In those months you had big sales. "sprzedałeś wtedy" = you sold a lot of milk that one time.