Megathreads are super powerful. I’ve found them to be really helpful when it comes to deciding things to buy during holidays and crowdsourcing deals, especially on Reddit (we recently had a festival in India called Diwali and here’s an example of a thread that crowdsourced deals & reviews from ecommerce websites). I rely on them to buy hings I didn’t think I would’ve needed otherwise.
We once had an unintentional megathread in our community chat. A community memeber was moving out from his house and he made a post saying that he was giving away some books. Some members expressed interest in getting these and others started posting about clothes, toys etc. that they wanted to give away. This became a mega thread and some of them met in person and gave away stuff. It was interesting to see the first couple of replies so we “stickied” the thread, which gave it some more visibility. But in a week we took it down because the content on that thread was time bound. However, this gave us a validation to add a tag, perhaps “Buy and Sell” or “Giveaway” so that the discoverability is easier.