I have a business model where I want to charge per lead/per enquiry a vendor gets on the website. What tools (preferably free/basic) can I use to keep track of the number of leads a vendor gets and automatically charges them at the end of the month for only that number of leads?
That's a very common model ... what's really new in that ?
That's not a business model. You're describing a service. Besides, as @Anshav pointed out, that there are hundreds of companies already doing this, using a free tool to deliver a paid service? C'mon. That's far from realistic.
Can you define your Business