The Realtor Ghost is a ghostly agent who sold the Firehouse to The Grungie family.
When the Ghostbusters were on summer vacation, the Firehouse was locked up and not occupied by anyone for awhile. As a result, even the supernatural viewed the place as a prime location to sell. A Realtor Ghost showed a family around and promoted it as a unique fixer upper. Foul Grungie wasn't too sure, due to all the work required, but the enthusiasm of Pig Grungie won out. The realtor had a sale.