You want a money cheat? Sure. This trick will require two tractor beams.

First, purchase some material, preferably the most expensive you can afford. Shoot for roughly half your ship's capacity.
Undock
Jettison your cargo
turn around, tractor the cargo, IMMEDIATELY switch to the other tractor beam, and activate it as well.
You will have doubled your cargo, provided both tractor beams were active when you collected the cargo.
Sell the pile, then repeat.