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There also appear to be MTurk "shops"--some of the people are clearly sitting at the same computer (some even used the exact same sign...)

Yeah I noticed that too. There's also some location inconsistencies!

Here as well, look at the closet on the right and the pictures on the wall.

That doesn't indicate a formal "shop" to me.

If you're getting $1 for each picture of a person holding a sign you submit, you might as well have everyone else in the room take a turn.