What type of riding do you plan on doing? If its mostly recreational on roads and bike paths, the Trek 7.2 should be fine. that's what my wife rides. If you'll be hitting the trails, the $200-$300 price point won't get you much in a new bike. I'd suggest used for a mountain bike. If you go used, craigslist tends to be pretty good. you can meet face to face, ride the bike to make sure it fits, etc. Ebay is a good resource, but only if you know exactly what you want.

As for the differnece in prices among shops, no telling why the bike is priced $50 different. My advice is to go to the local shop, ask if they will match the shop in NJ. Even if they don't, they may give you a discount on hemet, under-seat bag, etc. I'd tend to go with the local shop because it tends to be easier for tune-ups and repairs.