I like your idea!
There isn't really a good way to filter posts within a community yet on Lemmy, so I was worried about users getting flooded with buy/sell posts. There's a reason this community and r/Montreal has rules about buy/sell posts.
However I, too, do see an advantage in keeping the community united. We could have a weekly buy / sell thread for the greater Montreal region, as you suggested.
If people are interested, I could make a bot that automatically creates the thread for the current week every, let's say, Tuesday, and automatically pins it and unpins the old one. Let me know!