When the Crossroads changed their apperance, the wooden stick was the new starting weapon.
Being the first weapon given, it is unknown why it would be sold for 100 bananas. If anything, it should be cheaper as you can get it for free at the start of the game.
The major update replaced the Wooden Stick with a Trainer Weapon as the starting weapon and you cannot get it anywhere in the game. However, it still exists, since people still have it.