Hi,Welcome to NP
"iPhone" is a trademark
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No one is saying that you cannot register them, but there is a big chance for you to be contacted by Apple lawyers to give the domain, or a UDRP will be filled against you.
I mean if someone buys it, then its on them, but trademarked names are always a skip.Is it one of those cases of "buyer beware" if you choose to sell it?