From the Super Mario Wiki
Retrieved from "https://www.mariowiki.com/index.php?title=Storehouse&oldid=2509892"
The Storehouse is a shop in Bowser's Castle in Paper Mario, and is the last shop of the game. The storeroom is behind the western door on the upper floor of the first grand hall of the castle and is guarded by a bored Spiked Goomba. Initially, he won't sell any items to Mario, but if the plumber shows that he has Coins, out of boredom, the guard will turn the storeroom into a shop and allow Mario to buy and sell any items he wants. The Spiked Goomba guard will sell items to Mario at higher rates than other shops in the game, but will buy "useless" items (like Mistakes and Pebbles) and other attack items at decent prices.
Goombario's tattle on the shop owner states, "He's one of Bowser's followers. He got bored guarding the storage room and decided to start selling stuff. Wow, his job must have been really, really boring. I can imagine. You've gotta wonder, though... I bet if someone found out that he sold stuff to Mario, he'd get punished big time. Maybe he doesn't know who you are, though..."
Best selling prices