Question
What do you think these website founder have done to build their client base. Sites like Askfortask, Taskrabbit, needto, helpouts.google.com.
These websites have one common thing, helping individuals find a cheaper alternative to get a job done.
How about a service where it helps businesses post their services for free to reach larger consumer base.
1) Should you concentrate on building a customer base but there won't be any business for them to look upon
2) Should you concentrate on building client base, but there won't be any customers.
I am looking at this problem from a marketer perspective.
Answer
Find the market that is ready to pay for your service BEFORE you go build anything.
Go talk to your target market. Find out what they want. THEN build the site.
Nearly everyone does this bass ackwards, and creates nothing.