A koopman was a merchant, with the plural being kooplieden. Kooplieden could trade in all sorts of goods, ranging from spices from the Far East, timber from the Baltic or fur from the Americas. Others traded in domestic products like grain, cloth or glassware.
One of the nice things about having a koopman in your tree is that they left much more records than the average person: purchase contracts, partnership agreements, declarations of debts, powers of attorney and a range of other contracts regarding their business. You might be surprised to find out how extensive their trade network was. Merchants were also wealthier than the average person and more likely to have a prenuptial agreement or leave a last will.
All these types of records were created before a public notary. Before 1811, not all regions had public notaries so in that case you will find these records in the archives of the local courts, in the section for voluntary cases.
When business deals went sour you can also find them in court records, in the section for civil suits.