How did the guy at the LFS instruct you to cycle the tank?
If you haven't given the tank an ammonia source (fish food, shrimp, or even die off from the live rock) then your test readings should be "normal"... in other words 0.0 for ammonia/nitrite/nitrates. That's because it's still just a tank of salt water with no biological activity going on.
If there was a cycle, and it's over now, then you should see some measurable level of nitrates. Nitrates are the end result of the whole nitrogen cycle. That assumes though that the water you put in the tank had no measurable nitrates in it to start with. If you used tap water to fill the tank, then I wouldn't assume that nitrates in the tank mean you're cycled. They could've been in there from day one from your tap water.