Concerning the Europeans' payments to the African slave traders, the slave ships usually departed for Africa with a rather varied array of trade goods. Prices differed by region and time period and there was always some small concern that they wouldn't have on board what the African traders wanted. Obviously, back then, it was impossible to have recent and accurate information within a time period short enough to make adjustments to your trade supplies, and one African merchant might be interested in very different things than the next one 3 miles down the road (so to speak).

The most prevelant trade supplies were a bewildering array of textiles, firearms, gunpowder, copper and metal pots, pans and cups, various tools, iron bars and bracelets ('manillas'), buckets and other various metal objects (which were often smelted down by African smiths to make, well, whatever the hell they wanted).

Then there were the kauri shells, a rather common means of payment throughout some of the slave regions on the African west coast. The kauris were not unique to Africa either, being used in other parts of the world as 'money'. The Europeans obviously had no use for the kauris, but many African traders wanted payment in kauri shells, so the Europeans were forced to engage in multi-money systems to procure kauris for trade. The Maldives were a large 'procurement point' for kauris for the Europeans. The Dutch VOC ships would use kauris as extra ballast for their ships on the return voyages to and from the East, then sold in Europe for the slave trade on West Africa.

The book about the slave ship Leusden by Leo Balai makes mention of an extensive list of trade products listed in the administration of the VOC. The Leusden was a rather 'successfull' slave ship, having made nine full round trips on the triangle-route, before the ship foundered and sank at the mouth of the Marowin river on her tenth voyage. This means the Leusden's travels are relatively well documented.

In any case, the list of trade goods, which would've been very indicative for the Dutch West African slave trade in the early part of the 18th century, as mentioned in the book,

Textiles

Annabas/Fustian, a strong fabric often patterned, used for sheets, blankets.

Atlas, a fine silk; striped

Baftas, fine cotton cloth

"Broulissen", coloured linen from India

Caffa, decorated silk or cotton

Cassa, loosely woven fine form of cotton, often from the Bengals

Chits, a painted cloth, cotton or silk, made on the entire Indian subcontinent

Cora cloth, a fine woven silk

Guinees, a coarse and heavy cotton, very popular in Africa to the extent that it gains the nickname "negroskleed" ('negroscloth').

Lemianasses, European cloth, blue with white stars

"Linguettes", a type of cord used in the seams of clothing

"Perpetuanes", a very strong 'sergelike' fabric from Portugal

Platilles, a fine, veil-like fabric

Raw Guinees, unbleached cotton fabric

Saai (Dutch for 'boring'), a woolen fabric from Leiden's sheetindustry, a cheap and simple sheet

Salempouris, a textile named after Salempors or Serampore on the Coromandel coast.

"Sleepsheets", used sheets. For some reason the Dutch sheets specifically were quite wanted, even the French and English bought them for their slave trade.

Metal products:

Armbands, also called 'manillas', that were open on one side. Originally intended as jewelry, they were later also used as 'money' and as raw material as African smiths melted them down.

Firearms, estimates range from 180.000 to nearly 400.000 of firearms introduced into Africa yearly by European slave traders.

Copper bars, used by African smiths to make jewelry

Knives, rather self-explanatory :P

Copper or tin bowls, used for a variety of things (it's a freaking bowl)

Spanish bowls, more bowls, but smaller.

Other goods:

Alcoholic drinks! French brandy (eau de vie) was very popular, but Dutch brandy (jenever) also sold well. To this day, old Dutch jenever is still very popular in Ghana.

Gunpowder, not just for war but also as a general 'tool'. In Ouida (modern day Benin), the price of a slave in 1725 was equal to 300lbs (136kg) of gunpowder.

Beads, many forms of beads, self explanatory

Mirrors, highly regarded ever since the Portugese started the slave trade, remained popular throughout

Flints, for rifles

The Kauri shells

As you can see, it's a pretty extensive list. Having a large assortment of goods was a form of 'safety net' for the traders, because buying slaves wasn't always easy.

One interesting thing that keeps popping out is the prices for slaves. Even today, the end-of-voyage accounting numbers are impressive, with 6 digit numbers from the sale of a few hundred slaves.

This was very much a big business.