"I sell flowers."
FYI: Lots of people are suggesting 売ます instead of 売ります. The former is not correct (you're missing り!).
Second FYI: If your kanji (or non-kanji) answer is being marked wrong in a listening exercise: we are, as always, very sorry about the issue, but Duolingo's programming will not allow us to define more than one "correct" answer for listening exercises. The system selects them randomly from our written exercises, and we also can't control whether it selects the version with kanji from the new tree or the version without kanji from the old tree.
Would this be more of a habitual vs current type of distinction?
はなをうります - I sell flowers/I will sell flowers (I do, or plan to do, sell flowers)
はなをうっています - I sell flowers (I am selling flowers right now)
うっています seems more natural for the act, but maybe the context this sentence was made under makes うります suitable too.