Starting niche affiliate marketing site: Free for merchants to join but I want to charge % of sale - how much?

I am starting a niche affiliate marketing service.

I allow both affiliates and merchants to join for free.

I intend to charge the merchants a % per completed transaction.

What do you think would be a fair percentage?

Look at Ebay's fees. You can also look at what companies pay affiliates like what does Amazon pay when someone buys a book thru an affiliate.