How can I unlock an iCloud account on my iPhone?

Sorry, but Apple does not get involved in third party sales, so there is no receipt from a third party that would prove you own the device. In most cases it is the original purchase receipt from Apple or Authorized Apple seller that is required to unlock the device, because anyone can print out a receipt claiming they purchased the device from someone.

Be aware there are many scams of people selling stolen devices and they are not even the actual owner of the device. This would likely be the case for you if the seller has ghosted themselves and you are no longer able to contact them. If you can contact them, they can remove the device from their account, so let us know if that is a possibility. That is just one of the many risks you take when purchasing a used device from an unknown party.

The All Too Common SAD Reality of Buying … - Apple Community

Only the original owner can remove the account for you. Reach out to who you bought it from. How to remove Activation Lock - Apple Support