He claims that it will pick up a cable network, Root Sports Southwest, which carries Astros and Rockets games and I never miss a game.
Is that possible?
Doubtful. I mean, it's possible that there is a good streaming service that may cater specifically to Root Sports Southwest, but I wasn't able to find anything off the top of my list. Also, most cable network streams nowadays need proof of a provider contract, and I would be really sketched out if he provides you with that information, as it's most likely stolen from someone else.
In addition, it looks like there was some progress made with being able to sideload apps onto the Fire Stick (sideload meaning install applications that aren't in the Amazon store). This is problematic: if all the dude is doing is sideloading the Fire Stick and selling it as is, that's as bad as what he did to your WiiU. You'd need proof that he actually rooted it, meaning you'd want to see the board and also see the UI to confirm that SuperSU is installed (an application that can only be put on via root).
Due to your shady history with this guy, here's what I'd recommend. Go to Best Buy, Amazon or Office Max and just buy a Fire Stick (they sometimes drop as low as 25$ during crazy sales). Then hit up
http://www.aftvnews.com and get info. If you're familiar at all with Android, you can cross-reference against a list of apps to see what can and can't be sideloaded. If you absolutely need Google Services (so gmail, Play Store, etc.) then you'll want root, and then that decision can be made. However, if you just want access to other streaming services, you might be able to dodge that hefty pricetag and just do it yourself. That is, of course, if you're determined to get a Fire Stick, and this isn't just a passing fancy.
I hope any of this helped!