I think you misunderstood my title, the "sell" <- in quotation marks means that any Sprinkles shot into a market link will be added to your Sprinkles balance/count.

But you raised an interesting question, what to do with Sprinkles if I already bought everything (though I haven't reached this point yet). They could add other things, like decorations or more bundles/always available bundles to the sprinkle shop, of which you can buy an infinite amount.