If you are looking to build passive income or a business via plugins and app stores, here are some of app stores and plugin marketplaces I personally feel are ripe for the picking.
eCommerce & Website Builder
WordPress – https://wordpress.org/plugins/
Shopify – https://apps.shopify.com/
Squarespace – https://www.squarespace.com/extensions/home
Team Communication & Project Management
Slack – https://slack.com/apps
Atlassian – https://marketplace.atlassian.com/
Monday.com – https://monday.com/marketplace
Zoom – https://marketplace.zoom.us/
Customer Support & Communication
Intercom – https://www.intercom.com/app-store
Zendesk – https://www.zendesk.com/apps/