Shops and Secret Rooms Greed is encountered in count as Mini-Boss Rooms, and cannot be bombed out of. They also prevent any other Mini-Boss Rooms from generating that floor.
Once Greed is encountered in a Shop or Secret Room, Super Greed replaces him in subsequent Shops and Secret Rooms. After Super Greed is encountered in the same manner, they cannot spawn in Shops or Secret Rooms for the rest of the run.
If Isaac teleports out of the room after defeating Greed but without killing all of the Hoppers he spawns, Isaac can re-fight Greed for extra coins or copies of Steam Sale.
Greed spawned by blowing up a special shopkeeper can never spawn as Super Greed, even if another Greed was encountered before.
Greed has his own account on Twitter. This was uncovered after the community dug up a figurine of Greed covered in Sharpie marks during an ARG, which quickly led to his account. After guessing his password, and tweeting on it, the password was quickly changed and was then used to announce the arrival of Keeper.
The Seven Deadly Sins that these mini-bosses are based upon are, in Christian teachings, a classification of vices from which all others are derived from.
If the room Greed/Super Greed was fought in is re-entered a few times, rocks begin to spawn in the corners of the room along with a few cobwebs in the center.
If the game is quit in the middle of fighting Greed, upon resuming, the Secret/Super Secret Room will look slightly different and Greed may become Super Greed.