Use a free tool like Swappa's IMEI Checker to confirm it is actually blacklisted.
The is a unique 15-digit code used to identify your specific device. When a phone is "blacklisted," its IMEI is added to a central database shared by carriers worldwide. Common reasons for blacklisting include:
Once blacklisted, the device will be denied service on Verizon and almost every other major network globally. Exclusive Free Methods for Verizon IMEI Removal
If Verizon refuses to remove the blacklist and you can’t get a refund from the seller, you aren't completely out of luck.