You can create premium plans and give listing advantage based on selected plans. Example. Featured Listing will appear on top and have extra fields.
Grow the traffic your site and once site is popular, you can sell the ad space. many companies related to your directory niche, buy these AD spaces to display their Ads.
You make an events section on your directory site and promote events and also sale events tickets.
Initially you can do listing entries for all business. once your site have traffic, you can reach out to the added business through email or manual marketing. You can charge them to claim their listing.
You can allow the listing owners to add their product in the marketplace and grab a % of each. product sale.
Many companies offer partner program such as Amazon Associates, CJ, ShareASale, or Clickbank. You can promote their products and redirect traffic to them through affiliate link and get some extra money for promotion.
Each business on your site is a lead for you. You can understand their needs and provide them related extra services. Example: SEO, content writing, – Reputation management, Social media managements, website services and many more.
Once you start earning monthly income and you are confident on your skill for creating a directory site. You can sale your directory site on flippa.com, empireflippers.com or microacquire.com and earn a big amount.