SWN: Merchants of the Void
Perhaps most importantly, there is no such thing as interstellar bulk shipping in Stars Without Number. Even the largest spike drive ships have cargo capacities measured in a few tens of thousands of tons, while a modern oil tanker has a capacity of around 500,000 tons. Trying to run a meaningful trade in bulk commodities has been a lost cause ever since the jump gates failed and massive system-ship freighters could no longer make the jump between worlds. Such commerce can exist inside a system, with huge slowboats moving from planet to planet, but modern trade relies on carrying small loads of high-value goods or expert services. Most small merchant ships such as the ones a PC group is likely to own rely on in-system supply runs as their bread and butter. They cart loads of food, water, replacement personnel, spare parts, and other life support supplies out to remote asteroid mines, research stations, and extraction plants. The pay is rarely much, but such supply run missions are excellent hooks for dropping a group into the middle of some disaster. Trade beyond supply runs usually revolves around moving local specialty tech or products. Local art, exotic pharmaceuticals, high-quality technical specialties, or passengers with special skills who want to reach a particular world all make up this class of trade. For most purposes, it’s not necessary to really elaborate on the specifics of this, though it can add flavor to make up a product or two unique to a given world. -Kevin Crawford, Stars Without Number, 235.