I think it really depends on where he was selling. In downtown new york or London (or other really touristy areas) people often sell postcards and canvases with Banksy's work on it. The only difference is if my memory serves me right is that the art is PRINTED on the canvases, i've never seen it actually painted on it. I can't really tell for sure, but i think I would have stopped and asked about it. And of course, if he was selling outside the touristy areas I would have payed more attention and it is more likely I would have noticed it's not the usual cheap printed copies. I really appreciate his work so for sure I would have known those are Banksy pieces, and the fact that some of the pieces he was selling are lesser known ones, might have also been a clue.