I would go for option 3 with SSO:
- your existing users can go back and forth without the need of a new account
- you can use
store.example.com
as the domain of your store forum - you can make it visually appealing and more marketplace-like
- you can use banners in your existing categories to point to your store
- you keep your community focused without cluttering the conversation space with unrelated items that many would have to mute if they’re not interested
- using multisite configuration, there’s no duplication of work for upgrades and maintenance.