There are Pros and Cons for both the approaches . Build vs Buy is always a decision that requires considerable moderation

I am assuming that you are just starting your business , so with this assumption the following pointers may help
a) what is the value add you bring with your job board
b) what is the differential your job board will bring as compared to other job boards in the market

So you have to weigh in against the following
for Buy / reuse : no maintenance, can avail support , to some extent less effort and hence less cost
but then you will have a generic job board , duplicate of other job boards
For Build : Specific to your values,unique to represent your differential
cons: maintenance, support, cost and may be time to market

we can discuss this in detail if you like

Speaking from experience, when trying to launch a new startup company, testing your idea to know what works and what doesn't work is the most important thing. More often than not, the original idea you start off with isn't the right idea or really not what your customers may be looking for in your product. It takes a lot of time and iteration to truly learn what your customers want in a product.

Custom software is really expensive and "slow" to build and maintain and chances are that if you choose to learn what your customers like and don't like using custom software, you will burn through a lot of cash and time. It is best to reduce this cost of learning by using readily available solutions, learn from them and then when you have "cleaned up" your idea you can invest in custom software

I have some experience in this field. First of all, you would need a good website and a large number of people who would be interested in your product at least 10K in order to have a revenue of 100 people $ 50 = 5K profit