It doesn't help the resale value that's for sure. Strange Taxco mark, don't know that I've ever seen a piece without some other marking/s to accompany it. All you can do is test it, it could be alpaca, nickel silver, or (best case) sterling. Scary marked piece.
It's more than likely Alpaca since the test was inconclusive. Could even have been silver plated Alpaca. Like I said, I've NEVER seen a Taxco piece signed liked that and I've seen quite a few. I'll bet I still have at least a couple dozen Taxco pieces in my stockpile right now.