To start, wait until the mushroom appears, then drag it onto the sell box to earn money. Do it until you have 50 coins. Now you got the hang of it. Purchase the fire flower now. You can switch the rooms whenever you feel like it. Better powerups cost more to buy/sell, and take slightly more time to appear. Can you reach the golden king mushroom?
I was kidna inspired for making an idle game by "AdVenture Capitalist". All the sprites where downloaded from google search image.