How to safely destroy an AC Adapter?
Cut the cord off right where it comes out of the box with some sturdy kitchen shears. Take a photo- that will probably be good enough. That method was used for 'destruction' of non-compliant adapters that were considered unsafe. 

I can think of other ways of destroying an AC adapter, but the seller might not readily recognize the remains. For example, .45ACP slug, ceramic kiln, various machine tools etc.