How to remove iCloud from refurbished iPhone?

I bought a refurbished iPhone and have been in contact with the vendor, because I have trouble with charging. (it's probably only the cable!) The person told me before I am returning it, I need to delete my iCloud account from the phone and said their records show that iCloud is still on the phone. I had to send my IMEI no. I asked how can they see that, but didn't get an answer yet.

Could someone tell me how is that possible and if I need to be worried about it?

Thank you.

[Re-Titled by Moderator]

Original Title: iCloud still on my phone, known by vendor