Are you sure that you know which PC it's talking about? It's not in the PC where you keep your Pokemon. When you go underground using the Explorer Kit, you can talk to people there that will give you traps/items in exchange for spheres. If you don't have a Secret Base already, you can get a Digger Drill by trading someone with spheres. Then after you make a Secret Base, (if you haven't already) there will be a PC inside. Then if you access the PC and select "Decorate," the Munchlax Doll/Bonsly Doll/Yellow Cushion should be there. Otherwise, I have no idea where it is, and you cannot unfortunately sell a doll.