The economy is a commodity-based trading system. Planets will buy some products and sell others. Each empire will have common products (things bought/sold pretty much in every empire) and rare products (things that must be imported from other empires for a greater profit).
Planets are both the producers and consumers of goods. Ships negotiate with planets to purchase/sell items. The planet portmasters are very shrewd, so be on your toes. They don't like being insulted with low offers for their goods, or spending too much time haggling with you. They will also trade with more than one person at a time, so you should try to close the deal quickly if there is competition in the area.
Planets will renew their products once per day (most likely at something like 6am MUSE time). Once a planet has traded its cache of products, it will not have anymore to sell until the next day. So if you want something, better show up early!
Trade partners will be essential. An empire can survive in isolation, but to do so will be extremely difficult. On the other hand, trading goods across empires will net everyone a very nice profit and require alot less work. Goods can be traded at planets, or from ship to ship.
Planets will have both import and export tariffs. Tariffs can be set for an entire empire and/or for individual ships. Import and export tariffs do not have to be the same. A tariff of 999 disallows trading.