Use the public market

If you're primarily producing food or military units, then them to the public market at a fair price.

Use the money you've earned to purchase goods that your empire isn't specialized in producing.

Why use the public market?

1. You can usually earn more money selling to the public market rather than the black market.
2. You can usually save more money purchasing from the public market rather than the black market.

What is the Black Market is for?

1. To be available in case there are no goods available on the Public Market. The goods in the Black Market are usually priced higher.
2. To quickly sell something in case of an emergency when you need more money now, not later.
3. If you have enough keeps constructed, the prices on the black market may actually be better, especially if you've selected a race with a good market bonus.