If it's the same as the mineral town games, you win the Mystery Flower by beating Won at the apple swapping game(clearing the whole thing) which you do by activating the apple swapping event which unlock by raising Won's friendship level and entering the store when it's high enough. After that you can play the game just by talking to him(you have to pay him 100g to play, though). You can sell it as it is, and it sells for a pretty good price, but you're supposed to give it to the Goddess. She'll thank you for it and tell you to bring more...I think it's 9 or something, and when you give all of them to her she'll give you the Goddess flower which can be sold for a huge sum of money. You can repeat this as many times as you want, but I personally think it's more efficient to just sell the Mystery Flowers when you get them, it's a lot quicker.
Satoshi-kun wrote:^^ thanks!
I though I get those from the White Flower Event!
They do look the same. btw, you'll get more money if you just save them up and give them to the Harvest Goddess. Each sells for a maximum of 87,000G so if you get 10, that'll be 870,000G. If you get the Goddess flower, you can sell it for 1 million gold. You'll make 130,000 more gold if you do it that way.