You can buy Tritium. There used to be HUGE profits to be had BUYING LOW and SELLING HIGH, but I think that is gone now. Have to check INARA for places to buy it as it isn't available everywhere. To SELL it on a carrier, the Carrier owner has to put iin a BIUY order, but assuming the stocks aren't full, you can just DONATE (TRANSFER) it to the ship. At least that is how I remember it from when we went on our COLONIA jaunt.