So many things are unclear, what product category you are dealing with? You mentioned startups and businesses, this reflects some kind of intangible products and services you are dealing with? You said you have an option to charge the lead generations, this sounds like you have a B2B business model? If this is the case you better set your margins on sales done.

You can call me for follow up questions.

Thanks

Your target should be getting traffic to your marketplace and making prospects actually decide to use your service. This of course is done with proper marketing as well as having the right business model.
Try having a mixed business model where you provide a free listing that is basic and then provide paid services that boost the listing and its chance for successful sale such as Pinned Listing. Moreover, monetize ad spaces within the marketplace pages.

Well don't reinvent the wheel, take the time to research successful business model in your niche. Then read the comments for pro's and cons and go from there. You might need to offer both depending on what you are offering in your marketplace. Don't be a stranger if you need some more help.

Hello, I work with Digital marketers as their sales consultant. Most of them do PPC ads, AdWords, or generate leads through Facebook ads. They do pay per lead but they also work with clients one on one with a fixed rate.
For example, I have worked with some that are generating Chiro, plastic surgery, and law-related leads, and what they do is to drive calls to their business but will charge the business per lead.

Again it depends, you can also work with companies that will pay you to help them monthly on a fixed rate instead of a conversion basis.

Don't reinvent the wheel: check out your competition. What's working for them? (eg is your marketplace like sideprojectors.com or flippa.com? These are two very successful sites: is there a good reason NOT to use their business models?)

Let's have a chat about it if you'd like to discuss further.

If you are not involved in the valuation and/or due dilligence of any of this businesses, then you should only charge a % of the sale. In other words, if you are bringing together buyers and sellers, you could monetize by charging a "referral fee" that usually ranges between 3-7% of the closing price.