Forum moderation? - adding new?

[smr]

This might sound confusing.....
Can you add another modification option?
Image attached will explain.
In my forum I list eBay auctions.I list what it is, description and the link.
Rather than lock the thread when said auction is over I would like to select a 'SOLD' feature.
So instead of locking the thread, just select something that will show an image that has SOLD in it.
Sound possible?

[User 2877]

Yeah that's possible but will require a plugin and possibly core file changes. Certainly a few template changes too.

[smr]

So it is possible.Good to know.
Now to figure out what changes and additions need to be made.
Why would a plug-in be needed though? Could it not just be some additional PHP?