Example mount vendor

local T =
{
    {
        Name = "Ground",
        Token = 123123,
        Mounts = {
        --  {"Name", cost, mount},
            {"Name", 10, 123123},
        },
    },
    {
        Name = "Water",
        Token = 123123,
        Mounts = {
        --  {"Name", cost, mount},
            {"Name", 10, 123123},
        },
    },
    {
        Name = "Flying",
        Token = 123123,
        Mounts = {
        --  {"Name", cost, mount},
            {"Name", 10, 123123},
        },
    },
}

local function OnHello(event, player, creature)
    for k,v in ipairs(T) do
        player:GossipMenuAddItem(3, v.Name, k, 0)
    end
    player:GossipSendMenu(100, creature)
end

local function OnSelect(event, player, creature, sender, action, code)
    if(T[sender]) then
        if (T[sender].Mounts[action]) then
            if (playerHasItem(T[sender].Token, T[sender].Mounts[action][2], false)) then
                -- learn mount
                player:RemoveItem(T[sender].Token, T[sender].Mounts[action][2])
                player:LearnSpell(T[sender].Mounts[action][3])
            else
                -- not enough tokens
                player:SendNotification("Not enough tokens")
            end
            OnSelect(event, player, creature, sender, 0, code) -- show mount menu again
        else
            -- show mounts
            for k,v in ipairs(T[sender].Mounts) do
                player:GossipMenuAddItem(4, v[1], sender, k, false, "This mount costs "..v[2].." "..GetItemLink(T[sender].Token, player:GetDbLocaleIndex()))
            end
            player:GossipSendMenu(100, creature)
        end
        return
    end
    player:GossipComplete()
end

RegisterCreatureGossipEvent(123123, 1, OnHello)
RegisterCreatureGossipEvent(123123, 2, OnSelect)