How to remove iCloud account from used iPhone?
Take it back to the shop. Only the original owner can disable it, or Apple, if you can prove ownership &amp; sale by its original owner [which is harder than you think].

See How can I bypass Activation Lock?

If you can contact the original owner, then point them to this Apple web page -
What to do before selling or giving away your iPhone, iPad, or iPod touch

…see about halfway down, how they can do it after they already sold the phone.
Once they have removed it from their iCloud account, it will unlock &amp; you can set up as a new device.