Not true. The River Niger is navigable at considerable distances from the coast, as are several other rivers. West Africa is mainly low lying in the near coastal regions in most instances. The mountains of Nigeria and Ghana, two of the largest suppliers of enslaved peoples, are well inland.
The truth is that the African kingdoms were too powerful, and combined with susceptibility to tropical diseases and the high humidity battling these empires wouldn't have been possible. The Africans had immunity to various European diseases, so didn't collapse in the way that the various Amerindian empires of Mexico and South America did when the locals died from diseases brought in by the Spanish.
The wealth generated by slaves in the Americas and the importance of this slavery to continue to generate such wealth was such that if it was possible for the Europeans to reduce the cost of slave acquisition from African traders they would have done so.
Silly druggie is too unintelligent and ignorant to know that in the late 17th century and the early 18th Jamaica generated more wealth than did all of the British colonies in North America. And that Martinique generated more than did Quebec. And of course Ste Domingue (Haiti) was even wealthier than these.
I will watch him run to Wikipedia to post an article that undermines his argument simply because he cannot understand it.
Africa in from the 16th century to the era of modern transportation has been problematic. In the study of economics and trade in Africa; Fernan Broudel noted that
Geography is more important than history. Cultural exchange was even difficult for Africans. To the north was the vast Sahara. then there are the oceans on the other three sides. On the West in the sub Sahara regions, the smooth sloping coastline prevented ocean going ships from having natural harbor. For centuries much of the ship traffic bypassed the west coast. Ships of the era needed wind for their sails and winds were not reliable for the return trip to Europe either. It took later understanding of the currents to increase sailing ships here. Consequently little of African trade entered international commerce.
Consequently, before railroads and roads and plains, "external influence filtered only very slowly drop by drop". Escarpment, rift valleys and few navigable rivers affected development and this is besides the prevailing infecting and disease causing agents as the tsetse. The resulting isolating featrures of geography is seen in the Linguistic fragmentation of the region. It contains 1/3 of the language of the world even though it accounts for only 10 percent of the world population.
Also, despite being larger than Europe its coastline is shorter and there is a lack of coastal indentation, peninsula for natural harbors. Shallow coastal waters means ships anchor off shore and smaller boats used to ferry goods. Then there is the dearth of navigable rivers. Most river mouths are silted up or blocked by sand bars. The low irregular rainfall means that if one waits for deeper water one has only a short window to get up stream if possible.
No river in Africa except the Nile reaches far inland.Africa rivers that are seasonable navigable. These include the the Zaire river which while as prodigious as the amazon but only navigable short distance inland as falls and rapids block ocean going vessels up stream. A similar problem occurs with the Niger; it is not navigable by large ships and have and average dept of 4 meters in the dry season. Economically, much of what was in africa was not valuable enough to bring a return on investment except for Gold, Ivory and Slaves hence ivory and gold coast in West Africa.
You have to come up with rather strong reasons why trade did not develop to a high degree between Europe and Africa ( 16 to 19 century) given the interior possesses many other things the European needed than slaves.