Everyone who bought a piece is featured in that video. If you're going to buy one of these, you should probably make sure it's from one of those people.

Authenticating the provenance of Banksy stuff has always been a tough ask. The only way to definitively verify if streetart is genuine is to wait for it appear on his website, but for prints/canvases etc. he runs his own verification service [0].

[0] http://www.pestcontroloffice.com/