Duolingo says one translation of this is "the retail sale," which I would take to mean a single transaction. The online dictionary I use < http://www.dict.cc/? > suggests that the term refers to retail more generally--the retail trade, or the retail industry--but Duolingo gave me the Pink Flag of Wrongness for "the retail trade." Which should I trust here?
"Einzelhandel" is not used in the sense of a single transaction but rather refers to the industry. 'retail trade' is a good translation. I guess 'retail sale' was meant in the same sense. But it is misleading, I agree.