To be fair, there is a legal line connecting those dots, and there is a legal reason it doesn't matter.
The key is that the resale of like-new products is fine because it's not going to cause any brand confusion. The old iPhone you're selling is still an iPhone, and (assuming everyone plays by the rules) still has Apple's hardware inside the case. Legally, you're just selling an intact product (as you're entitled to under the first-sale doctrine), and you're not claiming to have changed its value in any way.
However, once you do something that changes the product's value, you have to be absolutely clear what that change was. If you replaced the battery or otherwise refurbished the product, you're supposed to declare that, so customers know exactly what they're buying. You could upgrade or overclock some components, and call it "modified", and even charge more for it. That's all fine, as long as you're making effort to say "this isn't exactly what that label might make you think".
You see, you are claiming first sale doctrine, but then you are completely undermining it. Once I buy something it is completely mine. Every part of it is mine. I can do whatever I want with it. Including reselling it.
You are talking about changing value and declaring it. What is the legal authority I should declare these things to? What is the law that mandates I do that? What even "value" means in this context!!
If you as consumer accept such erosion of the first sale doctrine, soon there will be no first sale doctrine.
The moment you remove the product from its wrapping, its "value" is diminished.
Apple may not allow you to resale product with diminished "value" that bears the Apple logo.
If you don't fight such interpretation, soon they will become the norm.
I can buy iMac, take its battery out and sell it.
All I have to do is say that it is a second hand.
Been second hand means I have bought it legally, I own it, and its "value" is diminished.
There is something more. The batteries are not visible part of the product. You cannot find out what is the logo on the battery of a working MacBookPro. You have to use special screws to open it and look inside.
The brand recognition is completely useless if it cannot be seen.
The only reason these parts have Apple trademark logo on them, is to claim counterfeit if they are been resold.