You just got your new smartphone and you are out to get a good case in order to protect it. Smart move Amigo! the new iPhone is ridiculously expansive and it is very important to keep your "investment" safe. There are quite a lot of different out there, some of them are good and some are plain horrible.
Picking a random cheap case on eBay is a bit of a gamble. I did it myself several times when I was in a hurry and just wanted to get a protective case fast and cheap. Some of the cases I got weren't bad for the price and did a decent job protecting my precious devices, but I always ended up getting a strong, solid and well tested case.
As I wrote before, there are a lot of different cases out there and if you are looking for a good one you better read the reviews. Try to go over as many reviews as you can and see what real people are writing about the case you are about to get.
You never know, you might just get a good deal after all, but if you had the funds and the time, you should always go for the well known, well tested case. This is the easiest way not to get disappointed.