> I have read of some vendors selling iPhones that would be unlocked at first but lock themselves to the first carrier they see.

Yeah, there's no official documentation, but that's how the Best Buy ones are. I've seen it called "US Reseller Flex" and "SIM Out" policies. There are some shady websites that have a GSX login that will report the policy name, anti theft lock status, and service history if you put in the IMEI. The intent is to prevent having to keep separate stocks of identical hardware pre-assigned to each carrier you sell for.

IIRC, Best Buy isn't supposed to sell you a phone without at least adding it to an existing carrier account. They act as an agent of the carrier, not selling the phone standalone like Apple does. It's possible that someone could then resell it while in-box as a scam, but that's not what Apple/carrier intended.