The OP specifically states that they know the technical part and are asking about the "financial side".

But - > "How do I build a service where I can link a buyer with a seller and earn money on that transaction?"

I googled that exact phrase just to see what came up, and the first link was this: https://www.sharetribe.com/academy/how-to-design-your-market...

Which highlights Paypal and Stripe as clear and strong answers to this question, which I'd expect most people asking it to already know - it's the other questions that are tougher.