You're probably running into this problem because Nigeria is known as one of several countries with a high risk of fraud. Here's a list of those countries "selected by the United Nations, world banks, the Internet retail industry, and OMIT as potential sources of credit card fraud."
OMIT Restricted Countries List
Many companies won't deal with those countries because of the high risk of fraud.
You'll have to look for a broker that is not located in the U.S. Do a search of the forums and you'll find many recommendations.