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This is bad advice. You shouldn't ask for IMEIs/serials, and you definitely shouldn't give them out if someone is asking you for one.
If I'm a bad guy with a stolen phone and I want to trick people into buying it, what do I do? I go and find someone with a legitimate phone and ask for their IMEI under the guise of making sure their phone isn't stolen. I now have a valid IMEI that I can use whenever someone asks for the IMEI to the stolen phone I'm selling.
if someone sells an Apple ID-locked phone on eBay, it's trivial to complain to eBay and get a refund. There is very little risk to the buyer in this case. Refunds in no way require the cooperation of the seller. If you're buying a phone on Craigslist, you're pesumably meeting in person and can verify the phone is functional and unlocked before any money changes hands. These easy solutions are greatly preferable to asking buyers for confidential information they shouldn't reveal (and which is meaningless if they do, since you have no way of knowing if they gave you the info for their device or someone else's.)