W czerwcu sprzedaliśmy rowery. (Mieliśmy dwa rowery i oba sprzedaliśmy).

Also, in a situation when "we" consisted of women only, it would be "W czerwcu sprzedałyśmy rowery", and the "were selling" version would be "sprzedawałyśmy".

"sprzedać" is perfective, "sprzedawać" is imperfective.

That means the first one is about selling something succesfully, the second is more about the process, about something that happens habitually. So this sentence sounds as if "we" worked in a bike shop in June and basically that was our job to sell bikes, while "sprzedaliśmy" would be just "We sold bikes" (probably our own bikes) and that's it. They're sold, we don't have them anymore.