Only the carrier to whom the phone is locked can unlock it - and they often won’t if you are not one of their existing customers.
Since the eBay seller is not returning your messages and the carrier appears to be unwilling to unlock the phone for you, the phone may be effectively locked to that carrier forever. You might want to try to return the phone for a refund - IF you can get the seller to cooperate, or if eBay has any buyer protection that applies here. Or you might be forced to chalk this up as an expensive lesson on the risks of buying a used phone.
It could have been worse; you could have gotten a phone that was either Activation Lockec or blacklisted. See:
The All Too Common SAD Reality of Buying … - Apple Community