>How do the people that post reviews, or spread something over word-of-mouth, discover the thing in the first place?

The follow industry conventions, visit registries of industry websites, have professional lists where companies submit their announcements (and not to the general public) and so on.

>Try your hand at starting a business and trying to sell goods or services using these methods and see how well it works.

If advertising is banned, it will work just as good as for any competitor.