OnAddVendingOffer

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

namespace Oxide.Plugins
{
    [Info("Vending Skin Guard", "RustPluginGenerator", "1.0.0")]
    [Description("Removes vending machine offers that sell items with non-whitelisted skins")]
    public class VendingSkinGuard : RustPlugin
    {
        private const string PermBypass = "vendingskinguard.bypass";

        private PluginConfig config;
        private HashSet<ulong> allowedSkins;

        private class PluginConfig
        {
            [JsonProperty("Allowed skins (0 is always allowed)")]
            public List<ulong> AllowedSkins = new List<ulong> { 0 };
        }

        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();

            allowedSkins = new HashSet<ulong>();
            foreach (ulong skin in config.AllowedSkins) allowedSkins.Add(skin);
        }

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

        protected override void LoadDefaultMessages()
        {
            lang.RegisterMessages(new Dictionary<string, string>
            {
                ["NotAllowed"] = "That skin is not allowed in vending machines on this server.",
                ["BypassOnly"] = "Only players with the vending bypass permission may sell this skin."
            }, this);
        }

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

        private string Msg(string key, string userId = null) => lang.GetMessage(key, this, userId);

        private void OnAddVendingOffer(VendingMachine vendingMachine, ProtoBuf.VendingMachine.SellOrder sellOrder)
        {
            if (vendingMachine == null || sellOrder == null) return;
            if (sellOrder.sellSkinId == 0 || allowedSkins.Contains(sellOrder.sellSkinId)) return;

            BasePlayer owner = BasePlayer.FindByID(vendingMachine.OwnerID);
            if (owner != null && permission.UserHasPermission(owner.UserIDString, PermBypass)) return;

            NextTick(() => RemoveOrder(vendingMachine, sellOrder));
        }

        private void RemoveOrder(VendingMachine vendingMachine, ProtoBuf.VendingMachine.SellOrder sellOrder)
        {
            if (vendingMachine == null || vendingMachine.IsDestroyed || sellOrder == null) return;

            List<ProtoBuf.VendingMachine.SellOrder> orders = vendingMachine.sellOrders?.sellOrders;
            if (orders == null || !orders.Remove(sellOrder)) return;

            ItemDefinition definition = ItemManager.FindItemDefinition(sellOrder.itemToSellID);
            if (definition != null) vendingMachine.RefreshSellOrderStockLevel(definition);

            vendingMachine.UpdateMapMarker();
            vendingMachine.SendNetworkUpdate();

            BasePlayer owner = BasePlayer.FindByID(vendingMachine.OwnerID);
            if (owner != null && owner.IsConnected) owner.ChatMessage(Msg("NotAllowed", owner.UserIDString));
        }
    }
}