Some islands can be bought undeveloped, while others already have roads and/or houses. Islands are also available for rent. Many celebrities have their own private islands. Commercial development of uninhabited islands can raise ecological concerns, as many have a fragile environment.

Krolow owns several islands in places like Canada and the Caribbean. He said the rentals of those properties act as a key revenue stream, forming around 25% of his annual income — the rest comes from commission he makes off of island sales.

You can find private islands for sale via Private Islands Inc. or Vladi Private Islands, or by contacting a realtor who specializes in big-ticket sales. Additionally, some high-end real estate companies, like Sotheby's International Realty or Christie's International Real Estate, list private islands for sale.

Sampson Cay, The Bahamas John Malone overtook Ted Turner as the biggest private land owner in the U.S. when he amassed 2.2 million acres in 2011. He now currently owns Sampson Cay, in the Exuma chain of islands in the Bahamas. The 31-acre island is home to a resort and marina, which was rebuilt in 2002.