Bikini Realty
The Bikini Bottom Realty is a realty that appeared in the episode "Opposite Day," when Squidward wanted to sell his house. Patty Rechid was also shown working for Bikini Bottom Realty in "Truth or Square"; when she sold SpongeBob a house.
Bikini Bottom Realty is likely responsible for all realty affairs because the company sold two of the main character's houses, most likely more of them.