If you do it in store, everything is on camera.
People who are looking to scam you won't do it in the store. They'll stop responding to you when you say you want to meet at the store.
If they report it stolen, you file a report and all the evidence you need is right there at the store.
Let's take it a step further, though and this may not be that common on this phone since it is relatively new but it's still very possible.
Seller A sells to Buyer B. But the IMEI number is still associated with Seller A. Buyer B for whatever reason wants to sell it. So he becomes Seller B. Sells it to Buyer C. That whole scenario you mentioned takes place in the store on camera and everything. Seller B is honest as can be and goes through all the steps to prove it's not a scam and the IMEI is legit and clean.
Now Seller A, who the IMEI number is still linked to, decides to report it stolen and they blacklist the IMEI. Phone shuts down all of a sudden on Buyer C. And only the original owner of the IMEI, Seller A, can have the phone turned back on. Not that guy who sold it to you in the store.
Which is kind of my deal. With the Sprint phones, which I used to have, you have to deactivate the ESN on your account before another person can activate that phone for their use on their account. There is no popping in a sim card and use. The seller has to deactivate the ESN from their account and the buyer then has to take that number and register it with customer service on Sprint. That right there makes the scenario I mentioned impossible. So if I check the ESN and it's still in use on an account, I know right then and there I don't want it. Or we can go to the Sprint store together and get it done.
Which is my whole point really. How can my scenario be prevented? How can the IMEI number be transferred to the seller? A clean IMEI is not secure enough. It could have possibly been stolen that day. And the person whom it was stolen from does not want to blacklist it yet as they want to wait a few days to maybe see if they can track it down.
Like the person who quoted me saying, yes this is way for people to shut their phones off when stolen. But like a lot of things, it can on the flip side be easily used for scammers to collect on insurance whenever they feel. You could be using your phone for months and months and they just decide to do it.