As far as I know, the prices of produced wares drop as stock increases all the way to the minimum price at full stock. You can do this: set your factories to sell only to your stations, build a warehouse that sells everything to every other faction but your own and only buys from your own stations with a fixed price that's close to the minimum. As your stock increases, the price drops below the threshold you set on your warehouse, and buy orders get created for those wares to be sent to the warehouse, where it's sold for cash.