Without knowing the AppleID/Password, you would need to provide your original purchase receipt to remove the Activation Lock. In most cases this is going to have to be the original purchase receipt from Apple or Authorized Apple Seller that shows the Serial or IMEI number. Apple does not get involved in any third party sales, as even I can make a receipt and it would not prove anything.
https://al-support.apple.com/#/additional-support
If possible, you may find it easier to gain access to your account again by resetting your Password. There are 3 ways that can be done and the first 2 will reset it immediately and if you have to use the last method of Account Recovery, there may be a waiting period.
If you forgot your Apple Account password - Apple Support