It would seem to me fairly hard to capture Africans in their native land, especially once word spread that Europeans were enslaving them. How did the natives react once the European intentions were made clear? Did slave traders have an army of sorts that just roamed the land capturing people from any settlement they happened upon? Did any African tribes put up any organized resistance?
African tribes were often at odds with each other and the practice of slavery was practiced in Africa (like in many other places and times). Surviving members of defeated tribes were often taken as slaves of the victorious tribe. It was from these tribes that the Europeans purchased slaves. As demand for a cheap and steady labor supply increased, the supply rose to meet it.
I don't think any Europeans blundered into the jungles of Africa looking for slaves, as this was the quickest way for them to get themselves killed. Instead, the slaves were captured/enslaved and taken to the coast and sold to Europeans who quickly loaded them onto their ships to be transported where they were most needed (read: where they would get the most profit).