The Ghost Sitter was the fifth story featured in the book More & More Tales to Give You Goosebumps.
Plot
Two siblings, Ray and Mike, have just moved into an old house their parents were renovating. At school, all day they are warned about the dangers lurking in the house next to their own. Then upon arriving home, their mother tells them they were enlisted to be the volunteers in helping to housesit for an elderly couple next door. Which surprises the boys since they were told by everyone it was abandoned/haunted.
Inside the spooky house, they smell a foul odor and they spot a bowl of gross slop inside a dumbwaiter. As they pour the gross stuff out, they make sure to pour it at an angle to avoid splashing any and making a mess. When they come back later to finish their chores, they discover that the house has been trashed by ghosts and they run out in a fright, taunting the ghosts.
Their hurried escape causes one onlooker to remark, "Hey boys, where's the Bonfire of the Vanities?" The elderly couple pulls up and the boys try to prep them for the mess inside by promising that they did not do anything while they were away, and that it was like that. But it turns out that ghost did not cause the damage, the couple's pet pig did!
The next day at school, the siblings brag about how nobody needs to heed what everybody else thinks about the ghosts or home, and that it is perfectly safe. The siblings are then informed that the house they are living in is the haunted one...