Just remember to check out the buyers history by looking at his rating (it's the little coloured star next to their name). That way you should be able to tell if you'll get a decent deal :)
Hope I helped
Try amazon, they usually back their products and so if you need to return anything they're usually pretty good about it. They also tend to have some of the lowest prices on the net.
Walmart and target always have great deals. If you want one even cheaper try searching ebay or craigslist for a slightly used one.