ReadMe.txt

Installation:
Apply the patch or the diff with git bash using command
git apply MultiVendor.diff
Supported TC version: https://github.com/TrinityCore/TrinityCore/commit/747184a7c0dc200b9c2c8607a3e1dd990161278e

Usage:
Set your NPC to have gossip and vendor NPCflags (129)
Add a gossip menu for him and add a new option to it.
The option needs to have option_id set to 3 so it acts as a vendor button,
npc_option_npcflag can be 1 or 128 (shows only if the NPC has vendor flag then)
and the action_menu_id is the vendor entry from npc_vendor that you want to show to the player.

YOU CAN also send menus from C++. All you need to do is to provide the vendor entry to the
void WorldSession::SendListInventory(uint64 vendorGuid, uint32 vendorEntry) function.

Command:
The npc add item command was modified so you can add items to multivendors as well.
The last vendor window must have been the multivendor you want to add your item to.
After opening the window you need to type .npc add item #itemId <#maxcount><#incrtime><#extendedcost> 1
The 1 in the end specifies that you are adding the item to the multivendor currently open.
Same thing works with .npc delete item #itemId 1

Multivendor.diff

diff --git a/src/server/game/Entities/Player/Player.cpp b/src/server/game/Entities/Player/Player.cpp
index 9ee7bc4..6ff16be 100644
--- a/src/server/game/Entities/Player/Player.cpp
+++ b/src/server/game/Entities/Player/Player.cpp
@@ -14358,7 +14358,7 @@ void Player::PrepareGossipMenu(WorldObject* source, uint32 menuId /*= 0*/, bool
                     break;
                 case GOSSIP_OPTION_VENDOR:
                 {
-                    VendorItemData const* vendorItems = creature->GetVendorItems();
+                    VendorItemData const* vendorItems = itr->second.ActionMenuId ? sObjectMgr->GetNpcVendorItemList(itr->second.ActionMenuId) : creature->GetVendorItems();
                     if (!vendorItems || vendorItems->Empty())
                     {
                         TC_LOG_ERROR("sql.sql", "Creature %s (Entry: %u GUID: %u DB GUID: %u) has UNIT_NPC_FLAG_VENDOR set but has an empty trading item list.", creature->GetName().c_str(), creature->GetEntry(), creature->GetGUIDLow(), creature->GetDBTableGUIDLow());
@@ -14551,7 +14551,7 @@ void Player::OnGossipSelect(WorldObject* source, uint32 gossipListId, uint32 men
             break;
         case GOSSIP_OPTION_VENDOR:
         case GOSSIP_OPTION_ARMORER:
-            GetSession()->SendListInventory(guid);
+            GetSession()->SendListInventory(guid, menuItemData->GossipActionMenuId);
             break;
         case GOSSIP_OPTION_STABLEPET:
             GetSession()->SendStablePet(guid);
@@ -21482,7 +21482,10 @@ bool Player::BuyItemFromVendorSlot(uint64 vendorguid, uint32 vendorslot, uint32
         return false;
     }
 
-    VendorItemData const* vItems = creature->GetVendorItems();
+    uint32 currentVendor = GetSession()->GetCurrentVendorEntry();
+    if (currentVendor && creature->GetGUIDLow() != GetSession()->GetCurrentVendorGUID())
+        return false; // Cheating
+    VendorItemData const* vItems = currentVendor ? sObjectMgr->GetNpcVendorItemList(currentVendor) : creature->GetVendorItems();
     if (!vItems || vItems->Empty())
     {
         SendBuyError(BUY_ERR_CANT_FIND_ITEM, creature, item, 0);
diff --git a/src/server/game/Globals/ObjectMgr.cpp b/src/server/game/Globals/ObjectMgr.cpp
index 04f3395..25a2244 100644
--- a/src/server/game/Globals/ObjectMgr.cpp
+++ b/src/server/game/Globals/ObjectMgr.cpp
@@ -8353,6 +8353,7 @@ bool ObjectMgr::RemoveVendorItem(uint32 entry, uint32 item, bool persist /*= tru
 
 bool ObjectMgr::IsVendorItemValid(uint32 vendor_entry, uint32 item_id, int32 maxcount, uint32 incrtime, uint32 ExtendedCost, Player* player, std::set<uint32>* skip_vendors, uint32 ORnpcflag) const
 {
+    /*
     CreatureTemplate const* cInfo = sObjectMgr->GetCreatureTemplate(vendor_entry);
     if (!cInfo)
     {
@@ -8377,6 +8378,7 @@ bool ObjectMgr::IsVendorItemValid(uint32 vendor_entry, uint32 item_id, int32 max
         }
         return false;
     }
+    */
 
     if (!sObjectMgr->GetItemTemplate(item_id))
     {
diff --git a/src/server/game/Handlers/ItemHandler.cpp b/src/server/game/Handlers/ItemHandler.cpp
index 3571410..d965a40 100644
--- a/src/server/game/Handlers/ItemHandler.cpp
+++ b/src/server/game/Handlers/ItemHandler.cpp
@@ -717,7 +717,7 @@ void WorldSession::HandleListInventoryOpcode(WorldPacket& recvData)
     SendListInventory(guid);
 }
 
-void WorldSession::SendListInventory(uint64 vendorGuid)
+void WorldSession::SendListInventory(uint64 vendorGuid, uint32 vendorEntry)
 {
     TC_LOG_DEBUG("network", "WORLD: Sent SMSG_LIST_INVENTORY");
 
@@ -737,7 +737,7 @@ void WorldSession::SendListInventory(uint64 vendorGuid)
     if (vendor->HasUnitState(UNIT_STATE_MOVING))
         vendor->StopMoving();
 
-    VendorItemData const* items = vendor->GetVendorItems();
+    VendorItemData const* items = vendorEntry ? sObjectMgr->GetNpcVendorItemList(vendorEntry) : vendor->GetVendorItems();
     if (!items)
     {
         WorldPacket data(SMSG_LIST_INVENTORY, 8 + 1 + 1);
@@ -748,6 +748,8 @@ void WorldSession::SendListInventory(uint64 vendorGuid)
         return;
     }
 
+    SetCurrentVendor(vendorEntry, GUID_LOPART(vendorGuid), GUID_HIPART(vendorGuid));
+
     uint8 itemCount = items->GetItemCount();
     uint8 count = 0;
 
diff --git a/src/server/game/Server/WorldSession.cpp b/src/server/game/Server/WorldSession.cpp
index 58e7ba0..45bcac1 100644
--- a/src/server/game/Server/WorldSession.cpp
+++ b/src/server/game/Server/WorldSession.cpp
@@ -121,6 +121,7 @@ WorldSession::WorldSession(uint32 id, WorldSocket* sock, AccountTypes sec, uint8
     recruiterId(recruiter),
     isRecruiter(isARecruiter),
     timeLastWhoCommand(0),
+    m_CurrentVendor(0),
     _RBACData(NULL)
 {
     memset(m_Tutorials, 0, sizeof(m_Tutorials));
diff --git a/src/server/game/Server/WorldSession.h b/src/server/game/Server/WorldSession.h
index 904006e..f448850 100644
--- a/src/server/game/Server/WorldSession.h
+++ b/src/server/game/Server/WorldSession.h
@@ -233,6 +233,13 @@ class WorldSession
         std::string const& GetPlayerName() const;
         std::string GetPlayerInfo() const;
 
+        uint32 GetCurrentVendorEntry() const { return GUID_ENPART(m_CurrentVendor); }
+        uint32 GetCurrentVendorGUID() const { return GUID_LOPART(m_CurrentVendor); }
+        void SetCurrentVendor(uint32 vendorEntry, uint32 senderGUIDLow, uint32 senderGUIDHigh = HIGHGUID_UNIT)
+        {
+            m_CurrentVendor = MAKE_NEW_GUID(senderGUIDLow, vendorEntry, senderGUIDHigh);
+        }
+
         uint32 GetGuidLow() const;
         void SetSecurity(AccountTypes security) { _security = security; }
         std::string const& GetRemoteAddress() { return m_Address; }
@@ -273,7 +280,7 @@ class WorldSession
 
         void SendTrainerList(uint64 guid);
         void SendTrainerList(uint64 guid, std::string const& strTitle);
-        void SendListInventory(uint64 guid);
+        void SendListInventory(uint64 guid, uint32 vendorEntry = 0);
         void SendShowBank(uint64 guid);
         bool CanOpenMailBox(uint64 guid);
         void SendShowMailBox(uint64 guid);
@@ -975,6 +982,7 @@ class WorldSession
         Player* _player;
         WorldSocket* m_Socket;
         std::string m_Address;
+        uint64 m_CurrentVendor;
 
         AccountTypes _security;
         uint32 _accountId;
diff --git a/src/server/scripts/Commands/cs_npc.cpp b/src/server/scripts/Commands/cs_npc.cpp
index a5b1330..e75509c 100644
--- a/src/server/scripts/Commands/cs_npc.cpp
+++ b/src/server/scripts/Commands/cs_npc.cpp
@@ -330,7 +330,8 @@ public:
             return false;
         }
 
-        uint32 vendor_entry = vendor->GetEntry();
+        char* addMulti = strtok(NULL, " ");
+        uint32 vendor_entry = addMulti ? handler->GetSession()->GetCurrentVendorEntry() : vendor ? vendor->GetEntry() : 0;
 
         if (!sObjectMgr->IsVendorItemValid(vendor_entry, itemId, maxcount, incrtime, extendedcost, handler->GetSession()->GetPlayer()))
         {
@@ -553,7 +554,8 @@ public:
         }
         uint32 itemId = atol(pitem);
 
-        if (!sObjectMgr->RemoveVendorItem(vendor->GetEntry(), itemId))
+        char* addMulti = strtok(NULL, " ");
+        if (!sObjectMgr->RemoveVendorItem(addMulti ? handler->GetSession()->GetCurrentVendorEntry() : vendor->GetEntry(), itemId))
         {
             handler->PSendSysMessage(LANG_ITEM_NOT_IN_LIST, itemId);
             handler->SetSentErrorMessage(true);