A handelaar was a trader. Handelaren could mostly be found in cities, not villages, and were often quite well-to-do.
Sometimes, specific words were used to show what people were trading in: a graanhandelaar traded in grain, a fietsenhandelaar traded in bicycles, and a handelaar in koloniale waren traded in colonial goods. You can also use newspapers, court records, or tax records to find out what your ancestor was trading in.
Another word for handelaar is koopman (merchant), where the term koopman is usually used for more affluent tradesmen.