I know mine got lost. Lost Hogwarts Acceptance Letter, customizable with your information, complete with your bedroom location in the house. I know a few people who would love to think that their letter just got lost and has finally made it to them.
Why would Hades sell Cerberus?<--- because after the Hercules incident, and then again with Percy Jackson, he decided that maybe a giant, three headed hellhound wasn't as reliable a guard dog as he thought.
Creation of the Marauders Map. >>>> It's because the map doesn't rely on vision. It shows people's physical place, which is still possible to find if you are under the cloak. Death could not find people under the cloak because Death was relying on vision.