how to return fake data on a repository method

I take it you don't want to use the fake backing store. That would be one way. It looks like you simply want to fake a repository on the client. You use a coroutine to do what you want. 

public IEnumerable<INotifyCompleted> GetItems()
{
    var List<Item> lst = new List<Item>();

[some iteration]

yield return Coroutine.Return(lst);
}

  public OperationResult<IEnumerable<Item>> LoadItemss(Action<IEnumerable> onSuccess = null, Action<Exception> onFail = null)
        {
            return Coroutine.Start(GetItems).OnComplete(onSuccess, onFail).AsOperationResult<IEnumerable<Item>>();
        }