When does an iPhones warranty start?

I'm looking to buy a BRAND NEW - IN BOX and SEALED iPhone off eBay, the seller has 950+ 100% positive feedback so I do trust them.

However, when would the 1 year manufactor / standard warranty start? Would this be when the phone is registered by me, or when the seller first received it?

Also, if there was a problem with the phone in the future, would I need a receipt as proof of purchase or would Apple look to see when I registered it?

Any help would be great, thanks.

Starts when the seller purchased it.
odds are unless its a iphone 5, the warranty is already expired.