If your imei is blacklisted then you have no chance removing that blacklist. If your imei is blacklisted then your phone has been reported lost or stolen and is now barred from service. Only the original owner can get that bar lifted. This is all from a UK user perspective but I would imagine its similar for other countries too.
People do this to do insurance fraud.
If you purchased your phone from a reputable second hand store and have the receipt then you should be able to get a refund.
I'm sure you will be okay especially as you bought it from a second hand store and I'm guessing you have the receipt. The situation would have been worse if this was a private sale with a stranger who can just go off the grid completely.
Best of luck with your situation. Your carrier should be able to confirm if your imei is blacklisted too.