So what’s the problem exactly? Apple didn’t prevent you from replacing the battery and the phone worked just fine afterwards.
Any iPhone that shows the warning message will be devalued on the second-hand market even if a user-replaced battery comes from a good supplier (like iFixit).
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As it should. As a consumer I would like to know whether a component of what I am buying has been serviced or replaced and who did the work. I'm willing to pay more for a used iPhone where the work was performed by a certified technician and I know that a genuine part was used.
No different than buying a car. I want to know if the transmission is replaced and if it was done by a certified mechanic at the dealer or by someone in their garage following a youtube tutorial.
I understand that's your preference, but I am speaking as a potential seller. I would never try to rip somebody off, but the problem with Apple's warning message is that it does not actually indicate the quality of the battery, or the quality of the labour that went in to replacing the battery. It just creates the impression of something dodgy.