Your question suggest an uninformed person should have some guarantee about the quality of the service they receive and have no regret. 

I can not think of any environment, in any discipline, under any condition where the uninformed have such assurance or guarantee or even expectation.

The folks you describe are 'next/last' decision makers.

They will call the last agent who contacted them or the next agent who contacts them. That IS the 90 day market and what keeps the companies in business who sell branded mailing services, internet ad placement, billboard, bus benches, grocery carts to real estate agents. 

How much of the market do you think comprises the 90 day market?

NOTE: many of the comments already offered RESULT in an informed citizen,therefore no longer a subject of this question.

Most think we are all alike, most often they learn the hard way when they dont get the service they need or want. They should interview an agent and ask what services the agent will do and what they may not.

Word of mouth (personal recommendations) remains the most potent influence for many buyers and sellers.

By height. You should always choose the tallest person available when seeking professional services. I thought this was common knowledge.

By interviewing them.  Or researching their interests online.  Like where they want to live.

It is all about relating to the client, giving them the confidence that you are the right person to hire.

Thy don't unless they search on the internet and find a Realtor that is all over the web...

they don't know who to call and certainly have no idea of whom to use... that's what marketing is for.... market to make the phone ring by a new potential client.... from there, the ball is in the agent's court to present them with the information of who they are; what they do; and how well they do it.... from there, the potential client will or should compare that info to what a few others have to offer....

The short answer, they don't know.