The pieces he sold here look intentionally mediocre. They're just reproductions using stencils he used a while ago on actual buildings. They look much better in their original context. Here they look flat and boring.

Banksy actually does some interesting stuff with paintings (http://imgur.com/S3CTKip) that's better than just using his old stencils on white canvases. My guess is he wanted these canvases to look like someone did the bare minimum to copy his most iconic stencils and make money off of Banksy's fame.

That said I would have liked to have bought one even if the real art is the entire performance and not the individual canvases being sold.