hello. i think this is very misleading of you again with regard to aller plus infinitive..
you ask : how to translate : he is going to sell his motorbike.
i would naturally want to use aller. so, il va vendre
but then you write [ to sell ] 'vendre' in le Futur Proche. strongly suggesting you want us to use the future tense of vendre.
but no, the answer you give is il va vendre, not the future tense of vendre but the future tense of aller.
this is very confusing.