Order by relationship column

join() worked fine thanks to @rypskar comment

$items = UserItems
        ::where('user_id','=',$this->id)
        ->where('quantity','>',0)
        ->join('items', 'items.id', '=', 'user_items.item_id')
        ->orderBy('items.type')
        ->select('user_items.*') //see PS:
        ->get();

PS: To avoid the id attribute (or any shared name attribute between the two tables) to overlap and resulting in the wrong value, you should specify the select limit with select('user_items.*').