I actually was interested in a Tag watch our local Costco was selling. I talked to my jeweler about it and he told me stores like Costco and Sam's club sell what he referred to as "grey market" watches. If I remember correctly, these watches were made overseas and the stores selling them might not necessarily be authorized dealers of the particular watch brand therefore the watch company most likely would not honor the warranty if issues came up.
I had never heard of this "grey market" (sounded to me like some sort of urban legend, haha) but he's usually very honest. My jeweler easily beat Costco's price.