At this point the best thing to do is just leave the fish alone and keep some Nori or Romaine on a clip and hopefully it will start to eat. Check your water conditions to make sure everything is on the up and up too.
A couple of things you should always do before purchase a fish. First make sure it's eating and not just brine shrimp. Make sure the fish is swimming and acting normal in the tank, not rubbing against rocks or hanging out near the top of the tank. Also check the color of the fish, it shouldn't be very dark or washed out. Finally ask how long they've had the fish. If possible you don't want to get a fish that just came in, as it's probably stress out, and now you're going to move it to another tank, which will only stress it out more.
Very important, you should Quarantine all your fish before you place them in your main tank.