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You're asked to deliver a specific quantity of <ware> to a station, but this order won't appear in the Trade Window until the mission is accepted. The station in question will probably have a trade offer up for the same ware, but the mission will not complete unless you fulfil the mission's demand (this prevents you auto-completing the mission with a suitable trade that's already on it's way: you'll have to set up or cancel & re-set-up a new trade to fulfil the mission's requirements).
If you don't have a Trade Computer, you'll need to fly within 500m of the trade port to uncover the buy order.
If you have (or can get) access to that ware for the mission's required amount through your own stations or discounts from NPCs (or simply stockpiling), take the mission first, then immediately set your tradeships to gather & deliver <ware> to take advantage of the trade bonus. If you don't do this immediately, the trade offers will update (as the mission committs you to fulfilling <ware> requirements, at a bonus) & you'll lose that opportunity..
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