Which countries became a major trading partner with west African empires in the late 1400s?

Only one country (namely Portugal) had any dealings with West African coast countries at the time. Portugal hoped to find gold there, and wanted to find the mystical Kingdom of Priest John, which turned out not to exist.

Calling Portugal a "major trading partner of West African empires" would be a vast overstatement. They got out some gold in exchange for iron tools, cloth etcetra but that was about it. Further travels along the West African coast by the Portuguese all had the purpose of finding a route to Asia.