The World Bank has come out with a wonderful short video explaining the benefits of trade liberalization among African countries:

Cato has addressed that topic in a 2005 paper:

[Accordingly,] in 1997 SSA countries levied an average applied tariff of 34 percent on agricultural exports from other SSA countries. Industrial countries, by contrast, imposed an average applied tariff of 24 percent on SSA agricultural exports. Similarly, SSA countries maintained an average applied tariff of 21 percent on nonagricultural exports from other SSA countries. Industrial countries imposed an average applied tariff of 4 percent on SSA non-agricultural exports.

According to the WTO, only 10 percent of African (including sub-Saharan African) exports were intraregional (i.e.: traded to other African countries). In contrast, 68 percent of exports from countries in Western Europe were exported to other Western European countries. Similarly, 40 percent of North American exports were to other countries in North America.