There is a lot of art out there, and a very small fraction of pieces are recognizable, even to art people. This guy probably got caught on the fourth try when someone did actually recognize the piece and say “hey wait a minute” and called the museum or something.

I’m not an expert but I believe you’re supposed to provide some kind of provenance. I’ve seen episodes of the original British antiques roadshow and tons of people had really old paintings just laying around. I suppose if you said you got it from your grandmother’s attic that might be good enough.