The motivation on the part of the exploited Nigerian nationals is simply to receive a few thousand (or even a few hundred) dollars, which represent much larger value in impoverished areas than those amounts of money do to the purchasers. On the part of the prospective purchasers, money is less often the issue because the main obstacle to receiving organs for transplant in more developed countries is the fundamental limitation on the source of such organs (Osita, 2003). Most developed countries have strict laws prohibiting the sale for profit of human organs and therefore, the main source of donor organs are cadavers who are typically accident victims.

While this explains the demand, the factors responsible for the ready supply of human organs in Nigeria (and other similar communities) is simply that public corruption has become epidemic and systemic within the local government and regulatory agencies...

In Nigeria, even the women rescued from forced sexual slavery situation are subjected to abuse while supposedly in the safe custody of the authorities.
Likewise, in the case of enforcement of human rights laws prohibiting medical exploitation such as organ trafficking, the available evidence strongly suggests that public corruption at every level from the licensing of physicians to the illegal transport of individuals and donor organs across international borders is responsible for undermining any efforts to address the problem. Therefore, the most effective means of eliminating such abuses requires a much broader focus on public corruption than on narrower enforcement of violations on a case-by-case basis.

References

Osita, A. "Corruption and Human Trafficking: The Nigerian Case" West Africa Review,

Vol. 4, No. 1; (2003).