First off, when buying iPhones off ebay, you should know if you want an unlocked one or not. If you have AT&T you don't need an unlocked one, and plus once you unlock an iPhone the warranty is voided. If you aren't an AT&T customer, you'll need to buy one that is unlocked, but I'd reccomend just getting a different type of phone for your carrier, or switching carriers to AT&T.

Jailbroken iPhones are iPhones where you can basically change the whole look of the phone, get free game downloads, and lots of other free applications.

When buying an iPhone, I'd ignore the ones being sold by users with a low amount of feedback (10 or less) and also the ones that don't include any pictures. Usually if they only have the stock picture in their listing they might be trying to hide something.

Personally, I'd look for ones that include the original box, because if there is an auction for just the iPhone & no accessories, there is a chance that is a stolen phone, which could lead to you getting into trouble.

Make sure you know what your getting. I've seen listings that say "Apple iPhone 8GB NEW Box" ...At first it seems like it's an iPhone with the box, but in reality it is just the box. I actually almost bought a buy it now item like that for $250 but someone beat me to it.

Make sure you know how many gigabytes of memory the phone holds. If the auction doesn't mention it, then I'd ask the seller, because a 4GB iPhone is about $250 less than a 16GB one.

Overall, the main this is pictures. If there are no pictures, stay away. If there are pictures, but none of them include the phone itself, stay away. You don't want to buy something, and then it end up being the wrong thing, because even if you DO get a refund, you have to wait almost a week to get the money back!