Well, if you bought it, you should be asking the person you bought it from. If it turns out to be stolen or lost, its very possible that the phone will be rendered useless if the rightful owner reported it to his carrier or Apple.
In Europe, it doesn't matter what carrier you choose if you have a legal phone. In the case of the iPhone, the carrier might need to have it unlocked for you if they are not the original carrier that issued the iPhone. That may require a fee.