The H seam on the back is the telltale sign of a legit sealed NES game. It's usually pretty easy to tell a reseal from an original. Originals are professionally done and sealed tight. Reseals usually aren't sealed tight, won't have the H seam, and can be identified by excess plastic tied off on the ends.
Your Dragon Warrior sealed looks legit to me, but it also happens to be the most common NES game to find sealed, since it was over produced, and didn't sell well, so they gave it out for free with Nintendo Power subscriptions in 1990. Unless the market has changed significantly since I last checked, it's worth about $10-$15.
Sealed NES games on the whole aren't that tough to find, and being sealed alone doesn't make a worthless game valuable. Most NES games can be find sealed on eBay for less than $20. I ran across a sealed Track and Field 2 at the thrift store last summer, and ended up opening it, since it doesn't have much value.