Actually, I don’t think that is a good choice either?
And Discourse does not have such functionality, nor am I aware of a plugin that does this. Discourse is able to delete replies after a certain amount of days (and TBH I think it’s one of its worst features, maybe only autobumping old topics is worse).
No, it doesn’t. It removes topics with no replies at all.
The obvious thing to do here is to have the topic auto close X days after the last reply / after creation or to manually close it after that chair has been sold. Keeping the topic gives people an idea of a) how to sell a chair b) what a chair is worth c) that a chair can be sold through the forum. A good example is the Marketplace category right here on meta.
This doesn’t make sense (at least to me) since the potential gain of #1 is less than the potential extra effort of #2.
Which can be prevented with settings like Max consecutive replies, Min post length and normal moderation.
Long story short, I would be reluctant to remove ANY topics (spam excluded), because the content is what defines your forum and its “culture”. It also breaks internal links, which can be frustrating to users.