What does IMEI mean when unlocking an iPhone?

So I need a new iPhone and am currently on sprint I'm still under contract until June. This guy is selling me an IMEI unlocked iPhone 4S for $200. Can I use the phone with sprint? And after my contract ends can I switch carriers to Metropcs he claims that because the phone is unlocked it is permanently unlocked and free to use with every carrier. Is this true? Am I missing any piece of information?

IMEI is the esn number for a phone and is use to factory unlock phones.
You can't use sprint with an unlock iPhone because its only unlock to any GSM carrier and Sprint is CDMA. But you can use the unlock iPhone on metro pcs. Yes if a phone is unlock you can use any carrier that use Sim Card(GSM).

Mark, that is not always true.

Robert: The IMEI number is like your ID number. It is unique for every phone and directly identifies the unit. In the IMEI there's a manufacturer and model info, it is coded right into the IMEI.

For unlocking see eBay or the link below.

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