Buying an iPhone 5s from a guy who is using it on AT&T. Will it work for me?

I'm buying an iPhone from a guy on craigslist. He's legit and is going to meet me at the AT&T store. He originally bought an iPhone on an installment plan and lost it without insurance. He had to buy a refurbished phone from AT&T and had to PAY IN FULL for it. From my understanding if I get the phone it can't get a bad IMEI for not paying because he technically owns the phone outright and owes nothing for the phone itself? (I'm aware it can be reported as lost/stolen and get a bad IMEI though). I'm going to try to have him request to have the phone unlocked from AT&T when we're there as well. My main concern here - is there any way his phone is attached to his AT&T NEXT plan? Correct me if I'm wrong but after I factory reset the phone it will no longer be attached to his NEXT plan… I believe the IMEI will stay under his name but if defaults payment on his NEXT plan (not installment) would the phone get a bad IMEI? I feel like it could only get a bad IMEI if he owed money on the phone and not due to not paying for the service. Let me know your thoughts! I plan to use the phone on Cricket by the way.

You need to find out if the cricket service uses the same technology as ATT, If they don't the phone won't work.

I would get the imei number from the guy and contact att myself about it - before buying the phone. You could also ask them at the same time if that phone would work with cricket.

Cricket uses At&t Service so you will not have a problem at all using the phone with their service at all.