jsは要素がdisable状態であるか否かを判断する

    //  button         
    if($("#buyButton").prop("disabled") == true){}

    //  button         
    if($("#buyButton").prop('disabled') != true){}

    /**
     *       
     */
    that.disableBuyButton = function(){
        if($("#buyButton").prop('disabled') != true){
            $("#buyButton").prop('disabled', 'disabled');
            $("#buyButton").css("background", "#8f8f96");
        }
    }

    /**
     *       
     */
    that.ableBuyButton = function(){
        if($("#buyButton").prop("disabled") == true){
            $("#buyButton").removeProp("disabled");
            $("#buyButton").css("background","#52a9e9");
        }
    }