I also have a plant that won't sell no matter what I do and wondered if anyone had found a solution recently. Unless there are "leaves" hidden on the "other side" of the plant, I don't see how that can be the problem in my case as, since it's a ball cactus, it's perfectly obvious where the leaves are. And anyway, doesn't the plant's health go down if it has dead leaves? My plant's health is always at 100%. I've also tried moving it around, lowering the price, selling it in a nursery full of other plants and so on. I've also sold several of this particular species before without problems.

Related question: do plants get too old to sell at some point, no matter their condition?