Core/Creatures: implement CREATURE_FLAG_EXTRA_NO_SELL_VENDOR

Port From (https://github.com/TrinityCore/TrinityCore/commit/f8dda8a4fc696d07bb6007e49140cec1b7f60dd5)

Source/Framework/Constants/CreatureConst.cs

@@ -212,7 +212,7 @@ namespace Framework.Constants
         NoMoveFlagsUpdate = 0x200, // Creature won't update movement flags
         GhostVisibility = 0x400,       // creature will be only visible for dead players
         UseOffhandAttack = 0x800, // creature will use offhand attacks
-        Unused12 = 0x1000,
+        NoSellVendor = 0x1000,       // players can't sell items to this vendor
         Unused13 = 0x2000,
         Worldevent = 0x4000,       // Custom Flag For World Event Creatures (Left Room For Merging)
         Guard = 0x8000,       // Creature Is Guard
@@ -234,7 +234,7 @@ namespace Framework.Constants
         Unused31 = 0x80000000,
 
         // Masks
-        AllUnused = (Unused12 | Unused13 | Unused16 | Unused22 | Unused23 | Unused24 | Unused25 | Unused26 | Unused27 | Unused31),
+        AllUnused = (Unused13 | Unused16 | Unused22 | Unused23 | Unused24 | Unused25 | Unused26 | Unused27 | Unused31),
 
         DBAllowed = (0xFFFFFFFF & ~(AllUnused | DungeonBoss))
     }

Source/Framework/Constants/ItemConst.cs

@@ -1144,12 +1144,13 @@ namespace Framework.Constants
 
     public enum SellResult
     {
-        CantFindItem = 1,
-        CantSellItem = 2,       // Merchant Doesn'T Like That Item
-        CantFindVendor = 3,       // Merchant Doesn'T Like You
-        YouDontOwnThatItem = 4,       // You Don'T Own That Item
+        CantFindItem = 1, // The item was not found.
+        CantSellItem = 2, // The merchant doesn't want that item.
+        CantFindVendor = 3, // The merchant doesn't like you.
+        YouDontOwnThatItem = 4, // You don't own that item.
         Unk = 5,       // Nothing Appears...
-        OnlyEmptyBag = 6        // Can Only Do With Empty Bags
+        OnlyEmptyBag = 6, // You can only do that with empty bags.
+        CantSellToThisMerchant = 7        // You cannot sell items to this merchant.
     }
 
     public enum EnchantmentSlot

Source/Game/Handlers/ItemHandler.cs

@@ -390,6 +390,12 @@ namespace Game
                 return;
             }
 
+            if (creature.GetCreatureTemplate().FlagsExtra.HasFlag(CreatureFlagsExtra.NoSellVendor))
+            {
+                _player.SendSellError(SellResult.CantSellToThisMerchant, creature, packet.ItemGUID);
+                return;
+            }
+
             // remove fake death
             if (pl.HasUnitState(UnitState.Died))
                 pl.RemoveAurasByType(AuraType.FeignDeath);