Single post in External links / Marketboard change

That situation isn't as hypothetical as you think. It happens all the time already with the Shelter and Wonder Trades. If you want to "be nice" then you're welcome to, however you are under no obligation to do so. Just like how if you find an accidentally-released Shiny in the Shelter, you aren't required to give it back. If a user pesters you about it, then that becomes reportable behaviour.