From the Super Mario Wiki, the Mario encyclopedia
Goombario's Tattle on the shop owner states, "That Toad runs this shop. He buys, sells and checks items. If you don't manage what you're carrying well, you may not be able to hold everything. Check items when you can."
When Shy Guys invade Toad Town, one of them takes over the shop and refuses to sell anything to Mario. Mario must whack the Shy Guy (either by jump or using his hammer) to repel him and thus allow the owner to reclaim his shop.
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