Defacing someone elses property by imposing your view of art onto it is vandalism.
Street art is something entirely different.
Personally, however clever and witty something is if it falls into the former grouping I think its antisocial and should have no place in our society. So Bansky vandalism is still vandalism. Banksy artwork on the other hand is clever, innovative and welcome.
As for selling out. Surely the whole concept of shared art is to deliver it to the widest possible audience. In fact Graffitti is really more guilty of this than most other art forms…..isn't it? So how is obtaining a mainstream outlet for his work selling out? Or transferring that to cycling should we all be riding self build bone shaker bikes with varying degrees of suitability for purpose??