The Arguim commercial outpost (factory / feitoria / fort) was the most important Crown-controlled locus of early Portuguese Atlantic slave trade. Its potential for sustaining and increasing the volume of slave exports has often been overstated, not taking into consideration its geographical and environmental marginality and logistical limitations (supply lines of all kinds). Juan Fernandez de Castros contract to purchase as many as 4,300 slaves from Arguim is a pertinent example of the resulting exaggerated and frustrated expectations. The limitations of Arguim are addressed in detail in Ivana Elbl, "The Slave Trade Logistics in Arguim, 1492-1519," in Manuel F. Fernandez Chaves & Rafael M. Perez Garcia, eds., El desarrollo del trafico esclavista en la modernidad. Siglos XV-XIX (Seville: Editorial Universidad de Sevilla, expected 2023). The space constraints of the chapter, however, did not permit a presentation of the full datasets derived from the slave purchase registers of 1508 (ANTT, Nucleo Antigo 888) and I5I9-I520 (ANTT, Nucleo Antigo 889), which provide detailed information on daily slave purchases over a number of months. The consistently documented purchase patterns offer key insights into supply, and the sex, age and value assessments of every single slave. The registers permit extensive dynamic comparison with the evidence provided by surviving maritime bills of lading (conhecimentos) that summarize the number, sex and age of slaves departing Arguim for Portugal by ship. The ledgers clearly demonstrate that the isolated desert-coast outpost of Arguim was wholly dependent on the willingness and seasonal readiness of regional (Mauritania, Western Sahara, and Sahel) suppliers to bring slaves to a point-of-sale (either Arguim or agreed-upon / conventional [expeditionary] locations in the close Mauritanian hinterland), and offer them for purchase either individually or in small groups.