Create a new version of paste:

using System;
using System.Diagnostics;

namespace FifthProject
{
    class Program
    {
        static void Main(string[] args)
        {
            Player player = new Player();
            Trader trader = new Trader();

            int id, price = 0, scale = 0;
            string name = null;
            bool usability = false;

            while (true)
            {
                ShowMenu();
                ShowBalance(player.PlayerBalance, trader.TraderBalance);
                int menu = ConvertToInt();
                switch (menu)
                {
                    case 1:
                        AddInfo(ref price, ref scale, ref name, ref usability);
                        AddNewItem(price, scale, name, usability, ref player.playerItems);
                        break;
                    case 2:
                        AddInfo(ref price, ref scale, ref name, ref usability);
                        AddNewItem(price, scale, name, usability, ref trader.traderItems);
                        break;
                    case 3:
                        ShowItems(player.playerItems);
                        id = ConvertToInt();
                        TradeItem(ref player.playerItems, ref trader.traderItems, id,ref player.PlayerBalance,ref trader.TraderBalance);
                        break;
                    case 4:
                        ShowItems(trader.traderItems);
                        id = ConvertToInt();
                        TradeItem(ref trader.traderItems, ref player.playerItems, id,ref trader.TraderBalance,ref player.PlayerBalance);
                        break;
                    case 5:
                        ShowItems(player.playerItems);
                        Console.ReadKey();
                        break;
                    case 6:
                        ShowItems(trader.traderItems);
                        Console.ReadKey();
                        break;
                }
                Console.Clear();
            }
        }

        static void ShowMenu()
        {
            Console.WriteLine("\tMenu\n" +
                              "1. Добавить новый предмет в инвентарь\n" +
                              "2. Добавить новый предмет торговцу\n" +
                              "3. Продать предмет\n" +
                              "4. Купить предмет\n" +
                              "5. Показать инвентарь\n" +
                              "6. Показать инвентарь продавца\n");
        }

        static void AddInfo(ref int price, ref int scale, ref string name, ref bool usability)
        {
            Console.WriteLine("Введите цену");
            price = ConvertToInt();
            Console.WriteLine("Введите размер");
            scale = ConvertToInt();
            Console.WriteLine("Введите имя");
            name = Console.ReadLine();
            Console.WriteLine("Можно ли использовать этот предмет?\n" +
                              "1. Да\n" +
                              "2. Нет\n");
            int menu = ConvertToInt();
            if (menu == 1)
            {
                usability = true;
            }
            else if (menu == 2)
            {
                usability = false;
            }
        }
        
        static int ConvertToInt()
        {
            int output;
            bool enterIsCorrect = false;
            do
            {
                string input = Console.ReadLine();
                enterIsCorrect = int.TryParse(input, out output);
                if (!enterIsCorrect)
                {
                    Console.WriteLine("Error. Try again");
                }
            } while (!enterIsCorrect);

            return output;
        }
        static void AddNewItem(int price, int scale, string name, bool usability, ref Item[] items)
        {
            Item[] tempItems = new Item[items.Length + 1];

            for (int i = 0; i < tempItems.Length - 1; i++)
            {
                tempItems[i] = items[i];
            }

            tempItems[tempItems.Length - 1] = new Item(price, scale, name, usability);
            items = tempItems;
        }

        static void RemoveItem(int id, ref Item[] items)
        {
            items[id] = null;
            Item[] tempItems = new Item[items.Length - 1];
            for (int i = 0; i < items.Length - 1; i++)
            {
                if (items[i] != null)
                {
                    tempItems[i] = items[i];
                }
                else
                {
                    tempItems[i] = items[i + 1];
                }
            }

            items = tempItems;
        }

        static void TradeItem(ref Item[] items, ref Item[] itemsSecond, int id, ref int balance, ref int balanceSecond)
        {

            if (balanceSecond >= items[id].Price)
            {
                
                balanceSecond -= items[id].Price;
                balance += items[id].Price;

                AddNewItem(0, 0, null, false, ref itemsSecond);
                itemsSecond[itemsSecond.Length - 1] = items[id];
                RemoveItem(id, ref items);
            }
            else
            {
                Console.WriteLine("Недостаточно средств");
                Console.ReadKey();
            }
        }

        static void ShowItems(Item[] items)
        {
            for (int i = 0; i < items.Length; i++)
            {
                Console.WriteLine($"ID: {i} Название: {items[i].Name} Цена: {items[i].Price} " +
                                  $"Размер: {items[i].Scale} Юзабельность: {items[i].Usability}");
            }
        }
        static void ShowBalance(int playerBalance, int traderBalance)
        {
            Console.WriteLine($"Баланс игрока: {playerBalance}\n" +
                $"Баланс торговца: {traderBalance}");
        }
    }

    class Trader
    {
        public Item[] traderItems = new Item[0];
        public int TraderBalance = 10000;
    }

    class Player
    {
        public Item[] playerItems = new Item[0];
        public int PlayerBalance = 1000;
    }

    class Item
    {
        public int Price { get;}
        public int Scale { get;}
        public string Name { get;}
        public bool Usability { get;}

        public Item(int price, int scale, string name, bool usability)
        {
            Price = price;
            Scale = scale;
            Name = name;
            Usability = usability;
        }
    }
}