Validating ideas and talking to customers in marketplaces

For marketplaces like Shopify or Wordpress or WooCommerce etc., how do you validate ideas? There's always going to be competition (and probably established big players), so how to determine whether it's still worth it to join the competition?

As for customers, how would you reach customers in marketplaces? They search only in these marketplaces, so I'm not sure how to even directly talk to them unless you've already invested in putting an app out there. It's known that you should talk to customers before building anything, so how would I talk to customers when they just typically search for an app in the marketplace?