How to know if an iphone is paid off completely?

I'm turning 17 and I have one year before entering college. My friend buys phones and then sells them on ebay. He is making a whopping 15k each year doing this. I thought I may wanna try. I sold my first two phones and made 380 profit after all the ebay fees and such. This never really came into mind until now but when I meet up with the person I check the entire phone from head to toe. But how can I know if the phone that I'm buying is paid off completely? Checking the imei is one way to go but online imei checkers are not a hundred percent accurate. And remember i only buy iphones and not samsung. I do not want to buy a phone that has not been paid off and marked as blacklisted. How do I know if the phone is paid off. Thanks and I will be sure to rate 5 stars for best answer.

The only way to know for sure is to ask the cell company that the phone is from. Unfortunately, as far as I'm aware, you have to be the account holder in order to get this information. If you're not the account holder, they won't give you any information.