Buy From Authorized Dealers Only: There are some great "deals" on the internet and in mail-order Ads, but be wary: Is the seller an authorized dealer of the product in question? If not, the product manufacturer may not honor its warranty through that dealer (and is not obligated legally to do so).
Also, if you are buying from a shopping channel, such as QVC, you will often notice that a representative from the company, whose product is being promoted, is usually onhand. This is one reassurance that QVC, or a similar shopping channel, is an authorized dealer for the product.
Avoid Purchasing Gray Market Goods: Some mail order and online dealers deal in "gray market" goods. This means that the dealer may be selling a version of the product that is not intended for the U.S. market. It may have a Canadian or other foreign warranty, have a different model number, and the instructions may not be in English. Once again, the manufacturer is not obligated to honor its warranties if its products are sold in this manner.
Make Sure Everything is in The Box: Make sure that all accessories that are supposed to be in the box are included. To find out what items are supposed to be included, check out the manufacturers' website, or consult some reputable online sites, such as those listed by Price Grabber. Also, shopping channel outlets, such as QVC are usually straightforward regarding what comes in the box or what else is included in your purchase.
Know the Return Policy: Make sure the return policy is posted on the web site or AD and that you understand the conditions. There is no automatic right to a refund if you are not happy, if the return policy is clearly delineated. If you choose to buy from an un-authorized dealer, you may be totally out of luck. Know who you are dealing with.
Be Aware of Shipping Costs: With ultra-low internet prices, many products appear at first to be great deals, but beware of the shipping costs. If that $700 AV Receiver is $800 at the local retailer, but the online shipping cost is $50, you might want to rethink your purchase in light of being able to go to your local retailer if you have any trouble with your purchased unit, rather than having to ship it back to the online retailer for another $50.
Paying With Credit Cards Online: When paying by credit card online make sure that any purchasing you do is through a secure web site, preferably with an SSL connection. You can also check for the Better Business Bureau Online Participation Program. If ordering by Phone, make you have the complete name of the person you are dealing with. Although online and mail-order credit card fraud is about the same as brick/mortar credit card fraud, it is always better to be safe than sorry.
For more tips on mail order and online shopping, also read 10 Tips for Shopping Safely Online by Andy O'Donnell, About.com Guide for Net Security.