bought a locked iPhone from the authorities, how to unlock?

Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Do this online at: Activation Lock support request - https://al-support.apple.com/#/additional-support Apple says, "Proof of ownership documentation is required," but does not detail what currently constitutes proof. Prior to this new online service Apple typically required the original receipt issued by Apple or an Apple Authorized Reseller and documented provenance (so typically it would be the original owner who does this, or a descendant with legal proof of ownership). This will give you access to the device but won't restore access to the account used to lock it. There is nothing that can be done to expedite an Activation Lock request.

Apple recommends you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support

If they will not reset it, you are welcome to contact Apple but I would not be surprised if they were very strict about what they will accept or not. Even if the authorities sold it to you, it could still be classed as stolen property.

Refer to this page for Apple Support features ➔ Contact - Official Apple Support

If the presented options do not resolve your issue, keep experimenting with any series of selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

This page has telephone numbers for contacting Apple for support and service ➔ Contact Apple for support and service - Apple Support

Nobody here can speak for Apple. I suspect they recognize that people may sell a phone to another user and have decided that it would be extremely unlikely that a thief would steal both a device and the original receipt. If you buy a phone from somebody else you would also ask to have the original receipt. They would at least need to remove the Find My setting but even then should the device become accidentally locked and you needed to reset it, you would need the account information or the receipt. Apple are therefore willing to accept the presence of a receipt of proof of ownership, or so I guess. The bottom line is Apple gets to decide what they are willing to accept. They are making a balance between people complaining about strict unlocking policies, and people used to be complaining that it was too easy for a thief to make use of a stolen phone.

Authorities could well be in possession of stolen items. They re-sell them legitimately but that doesn't mean the items isn't in essence a stolen item. The German authorities would have to set up a system with Apple validating resale of items.

I know this is frustrating but it is the way things work. If you think they should be done differently then suggest it to Apple and/or the German authorities.

Product Feedback - Apple

I have never heard of Apple doing that and just because they said Apple will do it does not make it so. All you can do is ask Apple and if they refuse, return the device to authorities that made the false claim.

Recycling is an option that the authorities can do.

Apple will unlock any device that is purchased from them as they do not get involved with third party sales. All you need to have is the purchase receipt from Apple or Authorized Apple seller and submit it with the link below to start a support request to have the Activation Lock removed.

How to remove Activation Lock - Apple Support

As for the environment, Apple will accept a device to prevent it from going into a landfill. They do not pay for the devices, but will accept them and tear down the device to recycle it responsibly.

National Services - Apple

It seems odd that the authorities would be selling iPhones that a buyer would not be able to use. They should absolutely refund your money for the purchase. If the phone is locked, then most likely they are not able to get into the phone either. If they could then they could just perform a Factory Reset on it and it would not be a problem for you.

Nobody will want to buy a locked phone. They will find it as useless as you do and will likely be asking you for a refund. Sorry, but that's the way it is. As for the ad being removed, I suspect the web site is trying to discourage thieves from trying to find sellers who don't know about the locking feature but are looking for a bargain without realizing they are buying a useless device.