Oh, I didn't realize there was a tradeable version of Momoonga. Maybe the prices came from a valid listing after all then, although I'd be surprised if somebody with a (presumably very rare) tradable Momoonga were to list it for only 15,000 flux!
Your response suggests to me that the data for the tradability of items is coming directly from the game somehow--I'm curious about this but I don't know what questions to ask!