A fascinating subject, but sadly one with very little in the way of historical sources. In the time period you cite, the only written source that I'm aware of being cited is Cosmas Indicopleustes, though I've never been able to find the actual passage. . . earlier we have Herodotus. Both of these sources are vague and at best second hand, very short on details, but amplified by scholars perhaps too eager to elaborate on early trade beyond available evidence (these are anthropologists/economists rather than historians, typically)

Dolfsma and Spithoven [2008] make a strong case that "silent trade" is at best too poorly documented to say much about, and may not have existed at all.

So we don't know that this happened at all, and if it did happen we don't have any good sources to explain how it worked.

Sources:

Dolfsma, Wilfred, and Antoon Spithoven. “‘Silent Trade’ and the Supposed Continuum between OIE and NIE.” Journal of Economic Issues, vol. 42, no. 2, 2008, pp. 517–526.

Swanson, John T. “The Myth of Trans-Saharan Trade during the Roman Era.” The International Journal of African Historical Studies, vol. 8, no. 4, 1975, pp. 582–600P.

F. de Moraes Farias. “Silent Trade: Myth and Historical Evidence.” History in Africa, vol. 1, 1974, pp. 9–24.