User blog comment:Ghaztpyre/I LOVE BUYBACK!/@comment-24145588-20140215175441

Each island can have one monster in the buyback slot: the one you most recently sold on that island. If you haven't sold any monsters on that island since the buyback feature was added, there will of course be no monster available for buyback on that island. It applies to any monster, any island, any level. And yes, when you buy it back, it will cost you the same money as you sold it for, and the monster will be of the same level as when you sold it.