When selling data remember also that just like trading it will add to your reputation with the controlling faction at that station. Sometimes this doesn't show up immediately, so if you are selling a lot of data pause occasionally and check the colour of the station icon in the HUD display. If it has turned green you've improved standing with them. This also flows on to your standing with the major faction to whom they have allegiance. Once you hit "allied" status with that local faction your rep won't increase any more at that station, so move on to another station controlled by a local faction that has the same allegiance. This way you maximize the value of your data profits by increasing rep in the most efficient manner. Better rep will produce better missions locally, and open up the restricted ships from the Feds and Empire.