See Dkb's Used Kiting Gear section
Which type of kite should you buy?
This primary depends on your experience, the kiting conditions, and your budget.
C-kites are loved by experienced kiters who enjoy their stability and predictability. In many locations it is essential to have a 5-line C-kite versus a 4-line for greater safety and relaunch capabilities. The newest bow kites with their amazing ability to depower quickly and relaunch easily, are being picked up by new and experienced kiters. Some people think that they are much easier to learn on as they are more forgiving. In any case, we are seeing new variations of bow and C-kites and a number of brands trying to create kiteboarding kites that combine the best features of both types.
The de-power feature is most important in gusty, unpredictable and changing wind conditions. New riders or riders in gusty conditions prefer the bow because they can create a greater range of power and thus maximizing or minimizing the kites resistance to the wind. Not only does this increase the range of your kite size during each ride but it also means that you can own fewer kites. Many kiters have a quiver (3) of kites. With a bow you could effectively get away with one or two kites when kiting in a fairly consistent location.
Buying kites can be very expensive, especially when paying for the newest technological advances. In general, companies are coming out with flatter designs such as bow kites and 5-line C-kites with more de-power than ever. Is it worth it? Definitely. The newer kiteboarding kite designs are making kiting safer and your experience more fun. Most people reviewing the latest kiteboarding gear claim that their performance has improved. If you don't want to pay the premium of buying the new kiteboarding equipment, consider buying used. Buying last year's technology is your best option to being just slightly behind the newest stuff without spending a lot of money.
There are many ways to buy used kites online such as ikitesurf or ebay but also consider buying from someone you know. This way, you can ask questions while blowing up the kite, and inspecting it for holes, leaks and general wear and tear. A much less risky approach.
Brand loyalty seems to be a big determinant of which type of kiteboarding kites people buy. There are good reviews on a number of well established brands such as Airush, Best, Cabrihna, Liquid Force, Ocean Rodeo, Naish and North . There are many more brands to choose from and the newer the company the more likely the prices will be lower. Whatever the kite, the fact that most people are generally pleased with what they have, means that there are a lot of good choices out there.