Buying an iPhone from the blacklist of an online market like eBay or Craigslist is the most annoying thing that can happen.
There is a solution if you are one of those people who purchased an iPhone or blocked blacklist. Nothing is missing because there is a way to remove the status from your iPhone blacklist and you can use it with different networks.
If you are 100% sure that your iPhone is blacklisted AND you know the reason, you can keep an IMEI checklist. But if you are in doubt, and more importantly, if you do not know your IMEI blacklist status, you should request a checklist for the IMEI blacklist here.