The Black Market is a unique Infrastructure building in Tropico 2 that provides a way to buy supplies for ships without needing to produce them. After each purchase, the per unit cost of supplies increases. Therefore, it is generally better to buy larger amounts or fill larger ships first before buying for smaller ships. Generally speaking, the black market is good early on as it can potentially pay for itself in the long run through early cruises.
Repeated application of the black market will cause prices to increase. The skill of traders working there determine the price; higher skill means lower prices. Also, prices tend to gradually decrease if no purchase is made for a while.