Sellers should work with a Realtor to protect themselves and to get the best possible marketing of their property. How many sellers are excellent negotiators, able to separate themselves emotionally from the transaction, are familiar with the contracts that are a standard part of all real estate transactions, have easy access and the ability to analyze their home and the market in order to price it properly, are prepared to handle showings etc? In my opinion it's like asking why would anyone use a barber or hairdresser, when they can buy a pair of scissors and/or an electric trimmer and cut their own hair? We all could, but very few of us do, and let's be honest a bad haircut isn't going to be nearly as consequential as a poorly handled real estate transaction.
Buyers should work with a Realtor for many similar reasons, a Realtor should be adding value to the transaction through their ability to correctly analyze the market and the true market value of a specific property. They have experience negotiating and a familiarity with the contracts used that far surpasses what 99.9% of all buyers have. They understand financing and can make certain the buyer is properly matched to the best lender. They have vetted resources such as home inspectors, appraisers (if necessary) surveyors, tradesmen, insurance people, real estate attorneys and more. Finally statistics show that Buyers who purchase directly from Sellers or without the help of a buyer broker tend to overpay between 2%-5% for the property their buying and my guess is this is directly attributable to their being too emotionally involved.
The challenge is finding a great Realtor as unfortunately there are simply too many poorly trained, inexperienced or simply lazy Realtors out there. Too many states continue to make it too easy to attain a license and in my opinion our association doesn't push back hard enough to insist on tougher licensing and continuing educational requirements.
For buyers or sellers reading this who are wondering how to find a great Realtor, I've attached a link to a fairly recent blog I posted on Trulia on this subject.
Great question and I've enjoyed reading what other have had to say on the subject.