These fragments used to be looted from NPC corpses when the Cries of Alderaan Story Arc was ongoing but are not obtainable anymore. Therefore, these may be of interest to collectors for a fair price.
Combining together all 4 pieces of the Rebel Tranmissions (1/4 to 4/4) and turning the combined item to an Imperial Coordinator was granting Imperial Faction points and a few credits to the player.