What to check before buying boxed iPhone?

If you don't trust the seller, don't buy it.

https://discussions.apple.com/docs/DOC-250001038

Neither IMEI or device serial number offer much assurance. Counterfeit iPhones can have serial numbers that match genuine devices. IMEI numbers can be permanently Black Listed by the carriers. Carriers share the Black Lists, so if recently stolen it might not be blocked yet, but could be in a few months.

A 14 day, no questions asked money back guarantee if you are not satisfied with the phone.

Having the IMEI really tells you nothing useful.