There is no way to fade markers through API.
However, markers can be simulated by creating Custom Overlay.
Custom overlay usually contains a div with the help of which opacity can be controlled by javascript or jquery.
In order to create effects or animations over Google Maps markers, we need a custom overlay.
The marker can be added to map and it surely makes optimized: false option
var newmarkerimg= $('#map_canvas img[src*="iconmarker "][class!="imageadjust "]');