Bidz is one of the world's largest online retailers of jewelry. They're a very large, publicly-traded company, and they offer 24/7 customer service along with buyer protection options.
"Bidz.com is the largest interactive online jewelry auction where auctions start at $1 every 5 seconds. Whether you are looking for an engagement ring, a diamond bracelet, a pair of ruby earrings, or a contemporary time piece, shopping with us will provide you with a wide array of jewelry, low prices, and the only interactive auction experience online. This is why millions of people choose to shop for jewelry at Bidz.com."Click on the banner for an excellent video explanation of how the site works, and to view thousands of items on sale:
They also operate "Buyz", where they sell brand-name jewelry at fixed prices (instead of using an auction format):