I wouldn't buy the tuner off of Ebay.
1. The unknown, we don't know if it works, if it's locked, and what tunes come with it, they may not work with your car.
2. You buy it off ebay you don't get free tunes for life that American Muscle offers.
3. If you buy off of ebay, your are going to spend 75$+ just to get another tune for your car anyways from a respectable tuner.
AM offers the best starter tunes for the mustang out of the box that I have seen. A lot of us once we modify more may switch to another vendor, but it's where you can start and then you can continue to ask for new tunes if you don't like the ones you have.
Cobb Access Port
MAPerformance Tune, CAI, and Intercooler
NGK 6510 Sparkplugs
Everything else is for show.