how to proof purchase of a second hand ipad

Only the original owner of the device can unlock the device, or get assistance from Apple to unlock the device. You or your relative cannot get it done. The only proof of purchase that Apple accepts is the original purchase for the original owner, nothing from you as a second hand owner. Without the original owner, the device is worthless to you.

Ask the seller to contact that person or give you a refund.

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If you mean activation lock then you can't, Apple will only potentially help the original purchaser of a device (and if they have proof of purchase e.g. original receipt with the device's serial number on it), they won't help buyers of used equipment. You will have to find and contact the original purchaser, or return it to the store that you bought it from for a refund

Unfortunately that is irrelevant, only the original purchase receipt is acceptable

I am afraid so.