But keep these tips in mind:

Know your seller: Amazon and Sears have millions of products on their sites, so the chances are good that the seller of the item you choose won't be Amazon or Sears.

Understand how the company and the reseller deal with problems: Ultimately, Amazon didn't get involved in the Perenson's HDTV sale, but on its site it guarantees "purchases from Amazon Marketplace and Merchant sellers when payment is made via the Amazon.com website or when you use Amazon Payments for qualified purchases on third party websites. The condition of the item you buy and its timely delivery are guaranteed under the Amazon A-to-Z Guarantee." (Read the entire Amazon guarantee policy at ".") The Perensons called Amazon to confirm that it would back the transaction, and the Amazon operator told them that they would be covered. Amazon didn't need to get involved in their case--but the family did have to deal with several weeks of inconvenience and uncertainty.

Be aware that different sites handle their marketplace sellers differently: Buy.com, for instance, instructs you to contact the seller directly regarding a return or any problem with an order purchased from its marketplace sellers, and says that the products need to be returned directly to the seller, not to Buy.com in the event of a problem. Buy.com's cobranded sites, such as its Wireless Store, have their own return policies. At Sears, Imran Jooma, president of eCommerce at Sears Holdings, told us: "We have a team of customer service representatives who are dedicated to helping all customers resolve any issues. We also help connect them directly with the seller to help them resolve the situation." However, the Sears.com Returns page states that items not sold by Sears or Kmart cannot be returned or exchanged at the buyer's local Sears or Kmart. Each merchant on Sears.com has its own return policy.

Be extra careful when buying HDTVs: Most sellers have even more-stringent policies for and/or refunds (and return shipping costs) than for smaller or less fragile products.