This is a very interesting question that deserves a solid answer.
As an active investor in buy and hold muti-family properties, I usually represent myself when negotiating an MLS listing, therefore I must interact with listing agents from star to finish.
My answer is: honesty. I can deal with agents that are not used to commercial transactions, lack of understanding the real value of a property and even ignorance of the market, but dishonesty is not acceptable and I will call them on it.
For instance, I constantly run into MLS listings that do not mention how many units the property has. This is a basic information that I need in order to do simple math and to determine whether I could be interested. Another sign of dishonest behavior is when a listing agent states "potential income" instead of real current income.
I was recently exploring the acquisition of a 10-unit apartment building in South Texas and asked for current numbers such as income, expenses, etc. The listing agent promptly sent me PDF that included current income Vs. 2-year old expenses. To make things worst, he conveniently forgot to include property taxes and management fees! Obviously, the numbers looked great at a first glance and I almost made an offer. When I politely confronted the agent with my findings, he disappeared faster than a pizza at a Weightwatchers convention!!!
Again, honesty is the top quality that I look for in a real estate agent, but there are a few others that are of utmost importance as well.
Communication - I am so tired of dealing with answering systems, corporate email accounts that are not checked promptly, text messages that remain unanswered for hour or days, and people who simply do not care because they "have the listing" and eventually they will get the 3% commission anyhow. Just a few days ago I tried to make an offer through the listing agent of a 25-unit portfolio but it was impossible to get her to pick up the phone or reply to my email. I ended up hiring a buyers agent to place my very reasonable offer. If the sale goes through, she will not make the potential 6% commission in a deal that is worth over $1 million USD. Guess what? This particular group of apartments has been on the market for over a year.
Deaf ears - Like I said before, I represent myself in most scenarios. I keep telling listing agents that I know the MLS listings in my area by heart, I review them twice a day so there is no need to put me on their e-mail system or to send me lists of properties that "match my criteria". They keep adding me and I get dozens of emails with the same info every day. Pfft.
Time-wasting habits - I appears that some agents don't realize that investors are more interested in numbers than "walking properties". Unlike home buyers, first we have to fall in love with the numbers and then the property, not the other way around. For some reason, an unfortunate majority of agents insist in property tours that only waste of time (mine and theirs) if the numbers are not right.
My $.02.
