What's important to you, Mary?
I think the essentials are: that the agent is qualified - they know your area thoroughly and know the properties well; that they are available - that they have sufficient time to devote to you; that you relate well to them and understand each other.
The overwhelming majority of problems in real estate are a result of people with incompatible communication styles. You get a person who's not a "teller" and a person who's not an "asker" and nobody knows what the other has in mind!
So, in reverse order - find an agent you have an easy rapport with, who has the time to devote to you, and is an expert in your area and with properties like yours and the one you want to buy.
All the best,