There's a red-velvet-ropelike air of exclusivity around so-called "flash" Web sites that sell discounted name-brand merchandise for a limited time. One must be invited to join (though invitations aren't hard to get); only members can purchase their heavily discounted offerings, including jewelry from leading designers; and they don't show up on comparison shopping tools like Froogle. Executives liken them to sample sales, which offer great deals for a short time, typically 24 to 48 hours. But many brick-and-mortar jewelers don't like that analogy—or the sites themselves. In fact, www.gilt.com attracts more people than your average sample sale or, for that matter, 10 average sample sales. It's an Internet phenomenon that boasts over 2 million members. Each week, its nine jewelry and watch sales draw from over 200 brand names and have sold jewelry pieces for as muc