A "listed" real estate property means that it's listed with a real estate agent or a real estate firm. An "off-market" real estate property is not listed with an agent. These properties are typically being sold directly by the owners and in some cases, they can represent very good buys for real estate investors.
From an investor's standpoint, off-market properties can be advantageous because there will be fewer competing buyers. When there are fewer interested buyers, the investor will have more leverage during negotiations. The buyer might be able to secure a deal that he or she wouldn't be able to get in an environment where more competition is present.
Owners sell properties privately for a variety of reasons. Sometimes, they want to save money on real estate agent fees. Other times, they don't want to disturb their tenants. For example, perhaps a commercial property owner does not want to alarm the current tenants, so he or she sells the property privately -- so as not to raise concern about the security of the location and the tenant's leases.
Investors may be able to take advantage of a good real estate buy if the owner is selling a property in need of repair, or when the owner is selling a property for a bargain because the owner needs to liquidate his or her cash as soon as possible due to financial problems. When a new owner moves in, he or she could scoop up a property for a bargain price, attend to needed repairs and sell the property for a profit later on down the road.
Whatever the circumstances happen to be regarding the off-market real estate property you're interested in, you will benefit from a deeper understanding of real estate law as it applies to sales and purchases. This way, you will cover your bases and reduce the risk of running into legal and/or financial problems later on down the road.
Source: Forbes, "An Inside Look At Off-Market Real Estate Investments," Ray Pressley, Dec. 27, 2017