If this is Activation Lock, it can only be released by the owner of the Apple ID holding that lock, or with sales paperwork and identification that only the original purchaser will likely have.
If this is carrier lock, it can only be released by the previous carrier; the carrier associated with the subsidy.
And the carrier may only be willing to work through the person that held the contract.
If the carrier does unlock carrier lock: How to unlock your iPhone for use with a different carrier - Apple Support
The folks at Apple will not assist with either case; not with a third-party seeking to remove Activation Lock, nor with a third-party seeking to remove Carrier Lock.
As the seller has seemingly disappeared, expect to recycle it.
Don’t like these answers? (Nobody likes getting scammed, either.) We answer these Activation Lock and Carrier Lock questions quite often around here... Poke around for the other discussions. The answers will be consistent.