Buy it!
"You won't regret this purchase, my man! Thank you! I promise you won't see another Pokémon like that any time soon!" As you walk out of the center, you hear the man speak to the boy you were talking to earlier. "Hey, kid, you want a Magikarp?! Ultra rare, and strong too!"
You exit the center and head to the entrance of Mt. Moon. It's somewhat intimidating. As you walk inside, you see several Zubat fly out and around.
Your Options
Keep going
Try to catch one
Dex it