2 difficult questions

Hi,

I have 2 very difficult questions regarding Iphones that I cannot seem to get resolved myself so hopefully someone here can help me.

If I bought an Iphone 4 from someone else, and something happens to this Iphone, that falls under its warranty, would I be able to take it to the Apple Store and get it replaced (the sim card wouldn't match the serial number on the phone)?

Also, does anyone know that I restore an Iphone 4 to factory settings, would I be able to activate it with any sim card? Even a sim card that's being used in another Iphone 4?


Thanks!