"We already have an agent, but we have not signed an exclusive contract" How to get that interview and listing?

[Silver A.]

Please tell me how would you handle this situation:

A buyer asks me to find something for them. I put an ad into local newspaper (or somewhere else, maybe direct mail). "We buy this in area x, tel 12345"
The seller finds my ad, calls me. I explain I am an agent and I have a buyer. After hearing the details about their property, I know it's not suitable for my client. That leaves me the option of becoming their agent, handling the sale of their property.

Then the sellers tells me they already have an agent, but they have not signed an exclusive contract.

(Basically they have stated someone else is already working on it, and since they stated something once, they start to believe it more and more, and will want to take on another agent even less.)

How could I respond to that, so I could get a chance to meet them at location, and then sell my service to them face-to-face, in order to get that client?

That happened to me last night. I asked how they chose their agent. "Oh, it's just someone I know." I wished them luck with the sale. There has got to be a better way to handle this.