How do I make sure that the iPhone I'm buying online is unlocked and that they are done paying it?

Hi, I had a question, I am going to buy an iPhone but it is used I’m buying it from online but I don’t know how to make sure it is unlocked and that they are done paying it also. Is there a way to know if it is completely payed and unlocked? They gave me the IMEI, will that work?

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