I got an Iphone from eBay and need to reset it without any passwords?

I got an Iphone from eBay and it is still linked to a icloud account and when I contacted the seller he did not reply so I waited 2 months and no reply came. I still want to keep the Iphone but I need a way to reset it without knowing the password for Icloud and any of the details. I also tried to put the Iphone into recovery mode but failed. I held the power and home button for 10 seconds and released the power button after 10 seconds but kept holding the home button. So now it has a black screen and it is being un-responsive. When I attached it to my computer and launched Itunes it recognized the the iphone was in recovery mode. I basically want to try and reset it to factory settings.

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