Trying this: 

...

        private const string VendPrefab = "assets/prefabs/deployable/vendingmachine/vendingmachine.deployed.prefab";

...

                    _vendingMachine = GameManager.server.CreateEntity(VendPrefab, _spawnPoint, monument.transform.rotation) as VendingMachine;


...

        private void CmdSell(BasePlayer player, string command, string[] args)
        {
            var item = player.GetActiveItem();
            var blueprint = item.IsBlueprint() ? (byte)1 : (byte)0;
            _vendingMachine.AddSellOrder(item.info.itemid, item.amount, -932201673, 10, blueprint);
            item.MoveToContainer(_vendingMachine.inventory);
        }​

The sell order is added, but the item will not move. No error. Ideas?

edit: It works after inserting an item manually into its itemcontainer, seems the inventory is not created.
edit2: works now:

after .Spawn();

_vendingMachine.inventory = new ItemContainer();
                    _vendingMachine.inventory.ServerInitialize(null, 10);
                    if ((int) _vendingMachine.inventory.uid == 0)
                    {
                        PrintToChat("It was 0");
                        _vendingMachine.inventory.GiveUID();
                    }​