Anyone who switched to a Iphone?

So I might buy my roommates Iphone from her and it is used. So I have a question how do you activate a Iphone? Like with a Android phone you have to clear the ESN before you can activate it if its been used before by someone else. ESN is like date basically. Is there something like that with a Iphone? Do you buy it as is?

They activate themselves once being set up. If all is well it should take a couple of seconds.

Easy, just take it to a carrier store and tell them you want to signup for their service, when you walk out it will be working. If 4S or earlier then make sure you pick the service its for (like cdma for sprint, etc). If 5 or later then they are cdma and gsm both so no issues with that.