Experience and compatability.
Ask to see the listings on other homes they have or had. Are there plenty of good pictures or just one?
Good pictures can go a long way to get a home sold for top dollar, especially in our internet driven society where buyers are scouring the MLS. What a let down to see a listing mentioning good amenities and not see any pictures of them....Also take a look at the description. Does it simply say something like "great house, 4BR, show and sell" or is there some creativity involved with a description that would make a buyer WANT to see it? All our listings are now on the internet. Some buyers choose the houses they want to see, then call their agent with a list.
Does the agent explain things so that you have a clear understanding of the process? Trulia is loaded with questions from people whose agents did not answer their questions and left them desperately wondering.
Does the agent have the experience to know the process and how to avoid the common pitfalls that happen in escrow? Good agents don't just take your listing and find a buyer. They must interact with other buyer's agents & know how to make your listing stand out on the internet and MLS. Most likely, another agent will bring in the buyer. Your agent will need to determine that the buyer can truly get a loan. This is a BIG stumbling block.
In escrow, your agent must know the timeframes and how to coordinate for inspections, appraisals, title reports, HOA, etc. There will be a lot of people involved in processing your sale and any one of them can cause a delay or a major problem. Your agent needs the experience to keep it going smoothly and forsee any glitches before they become major problems.
Lastly, DON'T pick the agent because they give you the highest price. This is an old trick called "buying the listing". Once you sign the listing agreement, you're stuck with them. They'll let you sit on the market with the high price for a while. Then, they'll talk you into lowering it. This gets you a much lower price in the long run. You only get one chance for a great "first impression" that will bring the best buyers.