What is the ruling on buying and selling environmentally polluting motorcycles?

The basic principle concerning all things is permissibility and lawfulness until evidence of prohibition is established. This is a manifestation of Allah's mercy and the ease of Islam. Accordingly, the default ruling for motorcycles is permissibility. Their pollution of the earth is not a factor that necessitates their prohibition, as many vehicles and factories pollute the environment, yet scholars have not ruled them unlawful for that reason.