if you add additional things to the topic, instead of creating a fresh topic
If you add new items for sale inside an existing topic, then you are doing yourself a disservice.
Only new topics get listed in the outgoing message digests. So by "reusing" an existing topic, you greatly limit the exposure of your new offer.
And for all other "edits" such as marking an item as sold, brining a topic back to the top doesn't seem beneficial.