Look at the Tips & Notes in the "House" module (the modules were changed after you posted this question), under the "Pseudo-Passive" heading.
In Russian, it is not uncommon to use a Subjectless 3rd person plural to express passive voice: Здесь продают лук = "Onions are sold here".
Literally, the Russian means "Here [they] sell onions", but there is no "they" in the sentence, so in translating it to English, you use passive voice, to make it "Onions are sold here."
If you want the English to actually mean "Where do they sell sell onions?", then you have to say "Где они продаю́т лук?" That doesn't use the reflexive verb - which is the point of the module.
Basically, your translation is what the Russian means but not what it actually says, because once you've translated it into English ("Onions are sold here"), you have to restated the English in active voice - except the whole point of reflexive verbs in this context is passive, not active voice. By trying to use active voice, you're missing the point of the module's lesson.
Understanding the passive-voice quality of foreign-language reflexive verbs is something native-English-speakers have a hard time with. We're used to using a verb with a predicate adjective or past-participle to do that. Also, the literal translation of reflexive verbs can be troubling: In a very literal sense, the Russian here means "Where do onions sell themselves?", which is not accurate either in English or in Russian. It's understood to be "Where are onions sold?" (verb + past participle) - which is also not a literal translation of the Russian. It's a mental adjustment English-speakers have to make in becoming fluent in many foreign languages, where reflexive verbs are frequently used to express what in English would be passive-voice structures.