public boolean sellItem(int itemId, int customerId) {
        if (customers.contains(customerId) && items.contains(itemId)){
            for (Item i : items) {
                if(i.getItemId() == itemId && i.getStockNum()>0) {
                    sales.add(new Sale(LocalDateTime.now(), itemId, customerId, i.getCost()));
                    i.sell();
                }
                for (Customer c : customers) {
                    if (c.getCustomerId() == customerId) {
                        c.charge(i.getCost());
                    }
                }
            }
            return true;
        }
        return false;
    }