I highly discourage the use of second hand probes. They are prone to mishandling and therefore become useless. If the probe tips are not stored in saline solution and dry out they are worthless. If the probe is offered to you stored in that solution, do you know if they weren't exposed to drying out and then just recently moistened just for the sale?
You can actually do more harm to your tank using a questionable probe. If you get a false reading and adjust your tank parameters based on the incorrect probe reading, the chance exists that you can wipe out your system. Not worth it IMO