When buying anything second hand there is often an element of risk. Security items are often sold when people upgrade to a newer model, but is it ok to buy them second hand?
It often depends on what the item is and whether it has been reset or sold properly. If for example you are looking to buy an up and over remote security door for your garage, then you need to ensure that you have the only access remote for it otherwise it will not be that secure.
Also you may not be able to fully test the second hand item until you have fitted it, making it too late to return if you have a problem with it.
When buying any security systems I would strongly recommend buying new unless it has been refurbished by a reputable company and they can insure locks have been changed or software reset to factory settings.