Activation lock with proof of purchase

The manufacturer of any item has no responsibility to a subsequent purchaser once it is sold on. Suppose, for example, you bought a second-hand Rolex watch, only to find out when you got it home that it was internally damaged in some way. Your complaint would be with the shop who sold it to you, not with Rolex. The Activation Lock is designed as an anti-theft device, to make a stolen phone unusable and therefore worthless: it has been successful in reducing thefts but there are still cases where a thief has sold on a locked phone to some unsuspecting purchaser.

In your case the original owner should have removed the lock, and the shop you bought it from should have made sure that he had. And they have to take responsibility for its not working, so your complaint is with them.

Apple are not going to go unlocking phones other than the original owner, or provide a bypass method: if there were a way of bypassing the lock there would be no point in having it in the first place.

On the other hand, you are not supposed to resell your device once you have it?

Of course you can sell on your phone or any other device: but if you have damaged it (and locking it it is much the same as damaging the internals in some way) then you would be committing a fraud if you don't make it clear.