Freelance business on a niche

Probably not. For one thing this already exists. Secondly it's harddd to build a marketplace. Third its a lot easier if you are well known in the copywriting world to do so.

The freelance marketplace space is quite crowded these days. A common pattern on Odesk, Freelancer and Fiverr is that people tend to hire devs who charge less, compromise with the quality of work which eventually leads to frustration for both the job provider and job seeker.

I find building personal network to be way more efficient than fighting for projects and reducing price.