So the question you may ask is what is a 29er, and why are they popular?
A 29er is a mountain bike with 700c (large road bike) wheels. This makes it so the bike can go over rougher terrain. It overall could be said that a 29er mountain bike, is the same as a regular mountain bike, just a bit stronger, and larger.
The perks of buying a 29er over a regular mountain bike is that they are currently very popular... they could be called the "it" in mountain biking. They have a higher center of gravity, and can go over harder terrain. The bikes can take a larger rider than many 26" wheeled mountain bikes. These bikes are like a military hummer vs where as the 26 mountain bike is more like a jeep.
Before I keep going here is a bit of information that most bike shops dont want you to know, you can use hybrid tires on these bikes!!! 700c tires fit perfectly on these!!! SAVE YOURSELF SOME $$$ AND DONT BUY 29er specific tires!
Now the downside that I have found. The bike is way harder to get started than a 26" bike. When you have off road tires on these, it is near impossible to ride distance over road. With the higher center of gravity the rider looses some maneuverability, these bikes are harder to turn sharp than a smaller wheeled bike. The bike is good on more overall flat land, when getting into very hilly terrain it is iffy because it is hard to get up some of those big hills.
A good rule before bidding, go to a local dealer, test ride, remember their price vs the ebay price. Sometimes its better to buy locally because of the warrenties that are offered by the dealers. Remember that even though some warrenties are offered overline you are going to be paying for the shipping to send it back and forth to have it repaired. Add up every cost that you may incur from buying offline, or buying used, and think about it before you bid, check shipping costs against the real cost of shipping the item, some seller will really tear into you over shipping to make up some money. Just always be weary when buying, remember people are out to make money, not to make you happy, if you want a good customer service experience, then go to the local bike shop, if you want it real cheap, then go online. Just remember that there are pluses and minuses to both.