And if they don't, they can delete the device from iCloud.com (Find My) to bypass the issue.
The buyer should know this and ask the seller to unlock their device(s). And if it's a legit seller, you can contact them afterwards and have them unlock it.
And iPhones still get stolen. As much as people may not like theft, it is less of a social ill than waste is.
If you want to deter theft, work towards a society that doesn't drive people to thievery, rather than trying to solve it with weak technical solutions that do nothing to address the underlying problem. Apple could be more effective at preventing theft by donating a billion dollars a year to fight poverty, although that would risk doing some good for the world.
That's... a take. It sounds like you don't personally believe in property rights, and therefore think solving global poverty (an intractable problem that has never had a meaningful solution in all of human history) is a better way to solve theft than just addressing theft directly. The weird thing is people steal even when they aren't experiencing poverty.
Theft is an unalloyed social ill, and the only examples of theft that can be remotely justified are contrived examples of theft under oppression of basic survival needs, which is not the context, socially or otherwise, of the places where iPhones and laptops are being stolen, so not really relevant. Theft is wrong. Theft is social ill that has far-reaching consequences and many second and third order effects in society. Ending theft is a good thing, and is more than worth the trade-off of the minimal additional waste (considering aluminum is very recyclable).
I allege that this is a take. It's possible to believe in property rights while also understanding that human desperation drives people to theft. The vast majority of people resorting to thievery were driven to it because of the circumstances of their life, not because they're doing it for kicks. In a battle between property rights and human rights, at some point human rights takes precedence. You don't need to go Full Communism, you just need to acknowledge that desperate societies are not polite or prosperous societies.
Property rights are human rights.
I have a human right to be secure in my person and personal effects, and a human right to enforce that security against transgressors.
Sure, then we can consider the category of human rights may contain (among other things) the right to personal property, as well as the right to be assured of your next meal, and there is no law of nature that says that rights cannot be in conflict. In a society of extreme scarcity, the latter may be unachievable. In a prosperous society like the one that I live in, the latter is entirely achievable, although sabotaged by short-sighted people who have yet to understand that treating crime at the source--human desperation--is more effective and efficient than treating the symptoms of crime.
No such right seems to exist, in my view. That said, we have a duty and obligation as a society to help out those in need, but this is not the same thing as this being a human right. That meal requires the labor of another, and there is no human right in my viewpoint that causes the enslavement of someone else.
Unfortunately, it doesn't matter whether or not we believe it to exist. If given the choice between crime and starvation, people will choose crime. One can believe in the inviolability of personal property all they want; they'll change their tune when hunger sets in.
> That meal requires the labor of another, and there is no human right in my viewpoint that causes the enslavement of someone else.
Sure. But nature is not so kind. You have food, and someone is coming for it. Your option is to either kill them or give it to them. If you would die without that food, then the moral choice is obvious. If you have so much food that it would be physically impossible for you to notice the loss of any of it, then the alternative is true.
Then the person who buys it from them is going to send it back, and they'll know next time, or however that gets resolved. If they're selling it to a reputable commercial entity, that entity will know it needs to be done, and will ask them to do it first. Every iPhone I've sold to a company in the last x years has needed this process done...