We probably could make the title un-editable after a set time but it would just result in lots of people asking me to edit things for them further down the line. As it is, we very rarely see people edit their title in a way that causes a problem so it’s definitely not worth the cost and time to add that option when we can already undo any issues with those who do delete the title or whatever.
We also can’t force people to stick to any particular behaviour without adding it to the T&Cs so technically if people want their advert/data removed we have to allow it anyway, although the only few people I’ve messages to ask if they mind putting a title back always say no problem they didn’t realise.
A ‘mark as sold’ button is the way to go. And people might ignore it, but that’s just how some people are. Sorry!