Imagine you are selling something-anything-on the open market. If we broke down the walls of time and space and put everyone who might be interested in what you are selling into one big room, it is obvious that the more people there are in that room, the better chance you have of not just selling, but selling at a good price. The fewer the people exposed to your product, the longer the odds of a favorable sale.
Now imagine that you are selling the very same thing on a desolate roadside where only a few passers by will ever see you. Anyone who stops and looks has enormous leverage. If you don't sell to them at the price they offer, you have no other options. It's them or the highway. They've got you.
Putting a home on the Multiple Listing Service puts your home in the proverbial crowded room. The MLS is, in a very real sense, THE market. It absolutely dwarfs the number of people who might casually pick up a supermarket home magazine, get their fingers dirty perusing the classifieds, or those who might happen to stroll by your home and see the sign. If you don't think the MLS is THE market, just try convincing an asset manager at a bank to not multiple list a foreclosure to get it sold. Or find me a divorce decree that mandates selling a home that forbids listing it with one of those useless, overpaid brokers. Both the judge and asset manager would agree that if the home isn't listed, it isn't truly for sale.
Now...even though 99.9% of brokers would agree with what I have just written, there are some brokers & agents, many of whom are here in the New York area, who would try and convince you that keeping it off the MLS is in the seller's best interest. These agents are trying for something we call in the industry a pocket listing. A pocket listing is the agent's little secret, and their agenda is one thing, and one thing alone: to sell the property to their own buyer and avoid splitting their commission with a cooperating broker. The metaphor is perfect: instead of your home being openly displayed "on the shelf," it is in the agent's pocket (think of a shady guy opening his coat, displaying cheap watches), hidden from public view, and only shown to someone who isn't working with another broker. Pocket listings are sometimes called a "office exclusives." The name is prettier, but the intent is the same. The object isn't exposure, but secrecy. Secret houses don't sell very well.
There is no scenario I know of where a a pocket listing is in the seller's interest. Exposure is limited, cooperation from brokers who might have a buyer is cut off, and possibilities are diminished. I have had instances where sellers have asked me to keep their home off the MLS because they only wanted me to handle the perspective buyers, but that thinking is mistaken. Committed buyers often have their own agent. Even those that don't have a buyer agent may know full well that I work for the seller and be unwilling to have me handle both sides of the transaction. It doesn't work.
The benefit of a pocket listing to the listing agent is twofold. If they do find a buyer, they don't have to split the commission. Is that buyer the best one? Who knows? Why should they care? If it closes, they made a double commission. They also have an opportunity to use the listing to pull buyers in that they'll sell another house to. They put an ad on Craigslist, jettison the buyers with agents, tell inquiring agents it is sold or off market now, and solicit all the other phone calls from uncommitted buyers to use them when buying a home if the pocket listing isn't to their liking. A wise seller would never allow their home to be used like this. There is nothing wrong with picking up buyers from a listing, but only if the agent has made a 100% effort to sell the listing first. There is nothing 100% about a pocket listing.
Buyers should also beware of a pocket listing. You are dealing with an unscrupulous agent whose motivation is money only, you have no agent representing your interests, and there may be less recourse if you have a problem or complaint with that agent. Practice business in the sunlight, and avoid the dark alley of a pocket listing no matter what side of the transaction you are on.