"Où vendiez-vous des citrons ?"
Translation:Where were you selling lemons?
I think "where did you sell" requires the passé composé in French ('Où avez-vous vendu) and "the lemons" in French would be "les" citrons, not "des." And sitesurf, I did not understand your response about "used to" as I understand that "you used to" do something is a legit translation of the immparfait.
http://alexandriaesl.pbworks.com/f/The+New+Fowler%27s+Modern+English+Usage.pdf Page 815: there is no mention of one form being definitely better than the other one, so the issue remains open I think and we cannot be wrong when we consider "use(d) to" as a full verb