Mal_ItemCoreEx_KeyItems

var malPICNT = DataManager.processItemCoreNotetags;
DataManager.processItemCoreNotetags = function(group) {
    malPICNT.call(this, group);
    for (var n = 1; n < group.length; n++) {
    var obj = group[n];
    if(obj.note.contains('<keyItem>')) {
        obj.keyItem = true;
    } else {
        obj.keyItem = false;
    };
};
};

//OVERWRITE Window_ShopBuy.prototype.isEnabled
Window_ShopBuy.prototype.isEnabled = function(item) {
    if (DataManager.isIndependent(item)) {
      var typeMax = $gameParty.getIndependentItemTypeMax(item);
      var typeCur = $gameParty.getIndependentItemTypeCur(item);
      var typeCurKey = $gameParty.getIndependentItemTypeCurKey(item);
      typeCur -= typeCurKey;
      if (typeCur >= typeMax && item.keyItem == false) return false;
    }
    return (item && this.price(item) <= this._money &&
            !$gameParty.hasMaxItems(item));
};

Game_Party.prototype.getIndependentItemTypeCurKey = function(item) {
    if (!item) return 0;
    if (DataManager.isItem(item)) {
    var count = 0;
    var items = this.items();
        for (var i = 0; i < items.length; i++) {
            if (items[i].keyItem) count++;
        };
    return count;
    };
    if (DataManager.isWeapon(item)) {
    var count = 0;
    var items = this.weapons();
        for (var i = 0; i < items.length; i++) {
            if (items[i].keyItem) count++;
        };
    return count;
    };
    if (DataManager.isArmor(item)) {
    var count = 0;
    var items = this.armors();
        for (var i = 0; i < items.length; i++) {
            if (items[i].keyItem) count++;
        };
    return count;
    };
    return 0;
};