I'm assuming he means its provider locked. In short, its a software lock on the phone that disables it from working on other service providers. It can be unlocked, but typically, it must be done by the owner of the phone. They would call into their service provider, purchase an unlock code, and use the code to unlock the device; allowing it to be used on any service provider from then on. Also make sure whoever you purchase the iPhone from deactivates "Find my iPhone" with their Apple ID so that they don't have a permanent connection to the device. (Find My iPhone settings do not delete on a device reset as a security feature and your device will permanently be linked to their Apple ID until they sign it on said device and deactivate it manually)
I always recommend that you meet to purchase the device at their service providers store. They would have a better understanding of how to do this and can also check the devices IMEI/Serial # for a lost/stolen flag.
If you don't think you can 100% guarantee any of the above is completed, it will quite literally be a waste of money and you should walk away from the sale.
1) Always meet at your, or the sellers service provider (Cell phone store basically)
2) Always make sure the phone you're purchasing is unlocked, if not, try and work it into the deal. The unlock fee is usually charged to the other persons bill.
3) Make sure the device isn't flagged as lost/stolen (This can be done by making sure you follow step 1. Service providers should have a tool to check the status)
4) If the device is an iPhone make sure Find My iPhone is disabled with their Apple ID.
PS: Unlocking features used online are risky. There are sites that work but be warned that if you input a code incorrectly too many times into an iPhone it will permanently lock. I can't remember off the top of my head how many times (I believe its 10?)
Edited by C0bra, 05 January 2018 - 03:08 PM.