Lacking a little info about what you're specifically building, however I'll share how I build YourSales.
YourSales is a platform for outsourced B2B sales. We provide curated, continuously-developing, and high-performing sales professionals for leadgen, inside sales, field sales, and business development.
It's a non-automated platform. Clients can't log in and select a sales pro, and sales pro's can't login to een assignments.
Regardless, I spent a large part of 2013 & 2014 proactively sourcing sales pros from everywhere. Also had many conversations with sales professionals to get them to register with data relevant to getting clients interested in their services. Registration is free of charge - all revenue comes from client projects.
Let me know if you have any questions. If appropriate you can book some time to talk at http://salestalk.yoursales.com.