Thinking about a few different models and companies that are already out there doing it.
1) Service Fee: this may not work for you if payment is occurring off of your platform, but if you are able to still be involved during the contract phase, a service fee (% of the total deal) can be applied to the total. Think of the service fee Airbnb charges to someone when they book someone's place on the platform.
2) Membership fee: charging a small fee to enable access to the best agencies out there. Having curated agencies without having to do all the vetting is definitely something of value that people and businesses may be willing to pay for. You could potentially also charge a service fee to the agencies because you are generating more value for them--a new channel of customers with little legwork required.
3) Advertisements: I would argue this is the least desirable business model--no one likes to be inundated with ads. But if they are relevant and you're getting a lot of traffic on your website, there could be opportunities to highlight some of the best agencies or relevant tools for businesses to take advantage of.
Hope this is helpful to you and I would be helpful to talk through this in more detail with you. I've been a strategist and business designer at Johnson & Johnson most recently, have mentored many new businesses and am a founder myself. Good luck!