The value of all farmsand rural real estate for sale in Mercer County, New Jersey, recently was $207 million, representing a combined 675 acres of land for sale in Mercer County. Mercer County ranks 13th out of 21 counties in the Garden State for the total number of acres of land currently advertised for sale. Of all locations in Mercer County with land for sale, Princeton had the most land for sale. Government and state-level services are the main economic drivers in Mercer County. Mercer County is New Jersey's 16th biggest county (228 square miles). It's located in the Central New Jersey region of New Jersey.