First, make sure to research more than one potential company to do business with. There are many, many gold selling websites out there so you have no shortage of choices. When you visit their site, try to get an overall feel for the professionalism they've put into it. A reputable WoW gold seller will usually have been around for a while and should have many customer testimonials prominently displayed on their home page.
Ask your World of Warcraft playing buddies for recommendations. Anyone who has been playing the game for any length of time will be familiar with the practice of buying WoW gold and should be able to refer you to one or more websites that have good reputations. If you can't find any recommendations this way, visit a WoW forum and look for threads on "WoW gold buying". There should be several. You can usually get a recommendation in this way. While not as good as a personal recommendation from friend, the WoW community is pretty tight-knit and players are usually more than happy to help other players out with recommendations and their own experiences.
Before making your purchase, navigate the company's website thoroughly. A professionally put together website is usually (although not always) a sign of a reputable company. A World of Warcraft gold seller that takes the time and expense to put together a customer-friendly website usually plans on being around for a while. Conversely, a "fly-by-night" gold seller will slap up a "down and dirty" site that may advertise great prices, but just doesn't look professional.
Lastly, check out the website's customer support. Do that have a live chat option where you can interact with a real person in real time (not one of those obviously automated chat systems)? If they do, that's a very good sign that the company has nothing to hide and wants to go the extra mile to make their customer's experience a positive one.
Buying World of Warcraft gold is something that should be approached with caution. To avoid getting "taken for a ride", WoW players should do their homework before deciding to do business with a particular gold selling website. This businessdieoehee with a disreputable gold selling business, it is important to follow some simple rules-of-thumb before making a World of Warcraft gold purchase. If you take the time to learn what to watch out for, your gold buying experience should go smoothly.