Rumours can be accessed from the Missions page, and will inform you of any rumours that are going around in your current sector.
You are not able to pick up rumours on every planet.
Rumours of a stock surplus or shortage will affect the quantity and price of the given commodity on the given planet.
If there is a rumour of a surplus, it is likely that stock will be high and prices will be low. These conditions will be great for buying.
If there is a rumour of a shortage, it is likely that stock will be low and prices will be high due to demand. These conditions will be great for selling the appropriate commodity for a handsome profit.
Rumours do not last very long, so when you find out about one, it is best to head to the stated planet straight away in order to take advantage of the offer.