in English if you say moto for motorbike we would translate bike they are both abbreviations for the same word
as we have no context, "eu estou vendendo uma moto" fits better also in Portuguese.
I agree with Leoroschi. The verb"sell" more appropriately describes, for example, what someone does for a living: "I sell motorbikes". If someone has a single motorbike which they wish to sell, they would say "I am selling a motorbike". If someone were to say "I sell a motorbike", this then raises the question of how many times they sell it?
is motorcycle not '''motocicleta''. a motor is also used in speaking language.