you assume it is always on. I have seen a lot of folks that are tin foil hat wearers over being tracked who refuse to use find my iPhone
and there is the issue that you can't guarantee that the serial you are being sent to verify is the device you will be sent. There is nada stopping me from telling you one serial that I know is clean and sending you another that is not. Just as there is nothing stopping me from getting a stolen phone without FMi on, waiting until the last minute and putting a burner account on it to screw you over.
Only buy in person and go through the wipe and setup process BEFORE handing over the cash.
If you're buying from a site such as eBay, you can request that the seller disclose the serial number to you before purchase. If they ship you a unit with a different serial number, you can then easily report it as fraud and get your funds back.
As a seller, you should ALWAYS include the serial number with the sale, as fraudsters are known to buy phones then send back a different broken unit claiming you sold them a defective unit. If the serial number doesn't match, you can prove what they did.
The third party MDMs let us do that as well, right? Like http://www.bushel.com