How about the one who happens to be looking at it the "closest" (according to the algorithms)? That would pretty much immediately turn into a "I see you can buy it for a lower price, how about I pay you to buy it for me instead" situation...
I suppose you could use lenticular screens like those used in some cars to avoid driver distraction[0 - old article but you get the point] or for the 3D effect in the Nintendo 3DS. This might be getting a bit silly though...