I had a system I built for a clients lead generation business. I asked if I could re-purpose the software and in exchange I'd provide the enhancements free as well as discounted development. He agreed. It was a basic system where a lead would come in for a geographical area, that area would have a vendor assigned, and an email would be dispatched. If you do the SEO and marketing right, the leads become pretty passive.
I improved the system in two ways:
1. My client generated monthly invoices. This was a problem since he had to chase people down for money. I setup a credit system where you purchased credits (leads) in advanced. I gave a 20% discount for purchasing 100 credits at a time. I also gave new customers their first 10 credits free. No obligation, no contract, not joking.
2. Email sucks. I integrated twilio and did direct call dispatching with numbers I controlled. I spammed the internet with these numbers. If a customer ever dropped off, I'd just find someone else. Rarely did I need to do that since the leads were so good.
This required a lot of work up front, but in areas where my marketing was dialed in already it just always generated income. This is the kind of stuff fluff articles like this never tell you, the actual specifics.
You could now take this basic model and apply to any industry that requires leads. Bonus if the industry is underserved, not huge, and filled with techno-peasants.