This needs to be modded up approximately 27 more times. Every reason which you have for your competitors not wanting to touch this niche is a reason for advertisers not wanting to pay to advertise to your users. Can you identify some local Nigerian companies which are making money, on the Internet, legitimately, from Nigerians?

If not then your model is to be one step ahead of the banstick on Google. It is not a pleasant place to be!

On the flip side, the site doesn't need to make so much money because the cost of living in Nigeria is relatively low.

Maybe he will have a lot of ex-pat users too? Lots of Nigerians outside of Nigeria. Seun, you could subtlety promote the Overseas category.. or you could have a small fee for posting in (parts of?) the Overseas category while leaving the rest free - similar to craigslist's "free except for" policy.

The overseas category was very popular at the time of this post, but it's been de-emphasized a little bit now.