How do you go about building a client base on a website where you help them?

It's the chicken or the egg question and in all truth you need both to grow each other. You need to figure out about the minimum level of businesses you need to keep customers coming back and back. Second, you need to offer those business a free trial period, say 3-6 months, so that you have the businesses that will attract the people, but the businesses are not throwing away their money on an empty project, while you are recruiting your people.

You also need to market yourself and start early, while you are still setting things up. Get the word out: make youtube videos. Ask other sites to attach a link to your site. Write on blogs. Leave links on blogs (it is a bit of spamming but if the blog owners don't like it, they'll delete the link). Even better if you can do a trade for service with those same blog owners.

Be creative and MARKET MARKET MARKET. There's an old Russian saying: not advertising your business is like flirting with a girl across a non-lit room.

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.

I see many dangers in your sentence "Help businesses post their services to reach customers".
To significantly increase your chances of success (and avoid extreme overwork which often causes burnout), I highly suggest you do two things :
1 - select a specific industry
2 - select a specific geographic area
The best parallel I can do is with Starbucks. It looks today as if they are everywhere. But in reality, they started small, tested and tuned their 'system' locally, and gradually expanded geographically.
Facebook did the same (started by only focusing on the academic market, with a clear focus on the 'dating' aspect, and then expanded).
In your case for example, your statement could be "Help accountants in Boston post their services for free to reach a larger consumer base".
Notice how you get instant focus with this simple change, you now know who to reach on both ends.
You can get even more focus by using a specific business size and smaller geographic area :
"Help accountants doing over 5M$/year in Cambridge post their services for free to reach a larger consumer base".
I clear message like this is very important in terms of customer acquisition because this level of specificity creates an 'this is for me' reaction, which instantaneously creates trust in your solution (one size fits all is dead).