Rack Rash

When you mount a module into a case, the head of the screw or bolt used to mount the module can scratch the faceplate of the module. These scratches are referred to as rack rash. You can almost never see it when you mount a module, as the scratches are behind the screw or bolt head, but nonetheless some will pay more for a used module that is unscratched. So buy a bag of plastic washers and put them behind the screw or bolt head just to remove another reason for someone to not buy your used module.