It's a ponzi scheme, the next person pays for the previous person until the price gets too high at which point the last person loses the majority of their money. BitPotato is all but dead now, if you buy a potato there you have a pretty much zero chance of anyone buying it from you.
Is there another site that should be avoided, that you'd care to share?
BitGrenade is the most well known clone of BitPotato and works in pretty much the same way with lots of alt currencies. I think most people are cured of these hot potato style sites now as there seems to be very few takers now. At one point potatoes/grenades were going for thousands of coins.
In general if the potential returns are clearly unsustainable then you should definitely avoid it, something that pays out double and as a result the price increases by 2.5 times it's previous price for the next person. It isn't going to be sustainable for long as it gets too expensive extremely fast.