Actually, I live in the Houston area too and can comment on those places in Harwin as well. One of my friends has done business with Falcon and has recommended it to me. I have personally used Houston Car Stereo on Hilcroft and Westpark and they're good too.
All the places on Harwin are mainly wholesale, but will sell to retail customers as well. The big places sell legit items - and most are authorized dealers for that product so you will be able to keep the warranty as well. This can sometimes be an issue with eBay items because usually the sellers are not authorized dealers so there is no warranty.
If the store isn't an authorized dealer, they may have some in-house warranty going on. Make sure and ask prior to purchasing. Whatever the case, it's definitely worth checking out - eBay prices are sometimes/usually higher than the prices I have seen there. And these items are brand new in the box (although I'm sure they have used ones as well).
Also, jugador is right about the haggling. No matter what anyone tells you - all prices on Harwin are negotiable