OnVendingTransaction

using System.Collections.Generic;
using System;
using Newtonsoft.Json;
using Oxide.Core;

namespace Oxide.Plugins
{
    [Info("Vending Owner Guard", "RustPluginGenerator", "1.0.0")]
    [Description("Blocks vending machine purchases when the shop owner is offline or lacks a permission")]
    public class VendingOwnerGuard : RustPlugin
    {
        private const string PermUse = "vendingownerguard.use";

        private PluginConfig config;

        private class PluginConfig
        {
            [JsonProperty("Only permission holders can buy")]
            public bool RequirePermission { get; set; } = false;

            [JsonProperty("Notify buyer when sale allowed")]
            public bool NotifyBuyer { get; set; } = false;
        }

        protected override void LoadDefaultConfig() => config = new PluginConfig();

        protected override void LoadConfig()
        {
            base.LoadConfig();
            try
            {
                config = Config.ReadObject<PluginConfig>();
                if (config == null) throw new Exception();
            }
            catch
            {
                PrintWarning("Configuration file is invalid, using defaults");
                LoadDefaultConfig();
            }
            SaveConfig();
        }

        protected override void SaveConfig() => Config.WriteObject(config);

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["NoPermission"] = "You are not permitted to use this vending machine.",
                ["OwnerOffline"] = "The shop owner is offline, this machine is disabled.",
                ["Allowed"] = "Purchase confirmed."
            }, this);
        }

        private string Msg(string key, string userId = null, params object[] args) => string.Format(lang.GetMessage(key, this, userId), args);

        private void Init()
        {
            permission.RegisterPermission(PermUse, this);
        }

        private object OnVendingTransaction(VendingMachine vendingMachine, BasePlayer buyer, int sellOrderId, int numberOfTransactions, ItemContainer targetContainer)
        {
            if (vendingMachine == null || buyer == null) return null;
            if (vendingMachine.sellOrders?.sellOrders == null) return null;
            if (sellOrderId < 0 || sellOrderId >= vendingMachine.sellOrders.sellOrders.Count) return null;

            if (config.RequirePermission && !permission.UserHasPermission(buyer.UserIDString, PermUse))
            {
                buyer.ChatMessage(Msg("NoPermission", buyer.UserIDString));
                return false;
            }

            var owner = BasePlayer.FindByID(vendingMachine.OwnerID);
            if (owner == null || !owner.IsConnected)
            {
                buyer.ChatMessage(Msg("OwnerOffline", buyer.UserIDString));
                return false;
            }

            if (config.NotifyBuyer)
            {
                buyer.ChatMessage(Msg("Allowed", buyer.UserIDString));
            }

            return null;
        }
    }
}