I doubt you will find many used. They are cheap and reliable so people will be using theirs (for the most part). If you have Prime for the free shipping you could not make much of a savings, after shipping, on a used temperature controller.
To compete with the new one I would have to sell so low that it would not make it worth my time to sell one used. For instance if it cost $10 to ship, I would have to sell at $10 or less. Because, at @ $20 total it is not too much a step for the buyer to buy the new one which could easily be returned if there was a problem.