The short answer to this is to do something for your potential client first.

This way you will position yourself as someone who is not just a taker.

You need to build a relationship with your partners, sending gifts always works followed by a call

There are some very specific strategies you can use which do go beyond a few quick typed answers here.

Good luck - call me when you need me :-)

In a recently previous life, my company worked with other businesses that helped recommend and sell our services. Some of the things we did was provide them with the service for free for a year, and kept it free for them so long as they brought in sales for us. We also provided commissions on a sliding scale.

Read a basic book on the subject so that you can ask specific questions. Affiliate will depend on how your customers and those of your affiliates can benefit from the cross promotion.

This book should get you started (I am not an affiliate of them) http://ow.ly/NJl7P

Why don't you engage your current Business clients/customers to see a) how they would promote your marketplace if they were offered an incentive b) If they would be apt to telling similar businesses that fit the marketplace niche? This way you are not going in guessing but have actual data on your target affiliate and customer.

From that you can pick from a variety of partnership or affiliate platforms to manage the activity and then you (or hire someone with this skill) to recruit affiliates or partners that would be a right fit.

I have more ideas - up for a call if you are:)