I'm currently developing a niche market platform. I will tell at a later point in time exactly what kind of services are offered, but technically it is a similar platform to Freelancer.com where people are offering services and others are seeking services.
I guess the tricky part is to get first enough service offers on the platform, before marketing for the service seekers. I would like to hear from you how you would do that, what kind of marketing channels you would use, etc. and how you would measure success, how you measure your efforts and how you charge.