Rita Perenson of Long Island, New York, wasn't sure. Last fall, she and her husband thought they had purchased a from Amazon.com. But the seller on Amazon's site turned out to be the online arm of a store in Nashville, Tennessee. When the HDTV arrived, its packaging was already opened, leading the Perensons to suspect that the item had been used by someone else.

The Perensons also discovered that the store's customer service line had more-limited hours than Amazon does. When they reached someone at the number, the rep asked them to ship the HDTV back to Nashville--albeit at the company's expense--so technicians could evaluate it and decide whether to offer a refund or a replacement set. The Perensons finally got the TV they ordered, but the experience was unexpected and unpleasant.

"The confusion is that you are on the Amazon site and the order is processed by them, but they are not [involved] in all aspects of the sale," Mrs. Perenson says.

The good news about large marketplace sites like Amazon.com, Buy.com, and Sears.com is that you can view millions of products--and their prices--in one place. Another convenience at Sears.com is that you often have the choice of picking up the merchandise in person at a Sears or Kmart store through the company's MyGoFer service. (For details, visit the .)