or maybe give the creator the option to just delete the thread and start a new one?
that would be nice, less complicated i think....
or if you admin folk wanna deal with it you could process requests to have threads deleted...

You are right
I guess putting only one thread per user seems a bit stupid.
Autopruning would work better.

Yea, well that doesn't solve the issue I'm talking about now does it.

I suggest something else.
Auto pruning of this forum.
Sales threads that are, say, older than 1 month with no posting activity, should be autopruned.

I agree.  It won't take long before a "recycled" sale thread becomes unreadable.

The topic title can be edited, so just replacing the title with "ALL ITEMS SOLD" would be enough

In the trading forum, only one sell thread per user is allowed. When an item is sold and the seller wish to put up a new item for sele he/she must edit their sell thread. This results in that all the old posts from the past sales are still in that thread making no sense at all.
Could perhaps the creator of the thread get a access to a function that deletes all the old posts in the sell thread?