Can I add a new payment method on a second hand iPhone
I tried add my personal credit card details as a payment method but it was declined, I bought the phone from an ex user.
Question marked as
Perhaps the unit was not readied for use by a new owner?
What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support
//support.apple.com/en-us/HT201351
That requires an Apple product/device to be correctly set
up and a new user account with new owner, put into it.
If that wasn't done by previous owner for you to set up
the new owner user account, see above article & work
backwards. You cannot use that device unless its done.
An Activation Lock among other security will engage
and further make the unit impossible; unless you have
contacted former owner to give you ownership.
Activation Lock for iPhone, iPad, and iPod touch - Apple Support
//support.apple.com/en-us/HT201365
IF you have proof of ownership, adequate for Apple retail
genius; & restore new owner access, it could be used.