These days buying used cell phones on ebay, craigslist and Offer Up has become a bit more difficult. With phones now being financed for up to 30 months with installment plans sometimes you buy a phone on one of these sites and you end up buying a phone with a pending balance on a payment plan. Other times these are phones that are marked as lost or stolen and are blacklisted. Either way the phone might not be usable.
It is very easy to either ask the seller for the imei of the phone or if you are in person type *#06# and it will provide the number for you. The website I recommend is imeipro they have a section to check your IMEI by most large phone manufacturers as well as the major carriers. Then you can have a better chance of not buying a phone that was blacklisted and has a clean IMEI