Bikini Realty
Bikini Bottom Realty 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.