iPhone warranty from 3rd party seller

All warranty go through Apple regardless if you purchased the phone @ an Apple Store or at&t Store. If you have your receipt great! When you activate your phone via iTunes, you're phone is automatically registered through Apple. Therefore, they can tell the day you purchased it by the IMEI number located in the back of your phone.

Yes.. Did they also provide you with the receipt? It's always good to have the proof of purchase, whether it was originally purchased by you or not. The phone regardless of the owner has a 1 yr warranty. There is Apple Care now available for the iPhone. It costs $69 for an additonal yr..then you will have a 2 yr warranty as oppose to 1. Check the Apple web site for details on certain states where Apple Care doesn't apply. Since I don't know where you reside, you may want to double check if your state applies in case you were thinking about the additonal year.

A few years back I asked Apple this question regarding warranty on a Powerbook. Their response was that it had to be purchased from an authorized Apple reseller. eBay not being one of them. However if they had AppleCare already then that could be transferred to the new owner.

That being said in regards to the iPhone they need to send the original receipt or the chances of getting any support on it at all are very slim.