If you can't afford to fly a potential buyer around the pattern, you shouldn't be in the business (my opinion).
My time has been in an o-360 172 and this one was a o-300 172. Wanted to see how much different the motor are.
So an aircraft owner or his broker should be on the hook for an unlimited amount of test flights? Everyone that calls gets a ride, without any type of commitment at all?
That's ridiculous, especially in your case. You're looking for a ride to satisfy a curiosity, not an interest in that particular aircraft.
It seems reasonable to me that if a buyer wants to fly an aircraft that's for sale, there should be an adult conversation that goes something like "Now that I've examined the aircraft and log books, I'm interested in making an offer."
"I understand the owner might have reservations about test flights, so how can we make this happen? I can confirm my insurance will cover me, I can also pay a reasonable amount for fuel and other considerations. Is this acceptable?"
Sounds a lot more responsible and accommodating than than a phone call saying "I won't decide to buy it unless I can fly it". The broker doesn't owe you squat, you're just a guy on the telephone.
Unless you actually exhibit a real interest in the aircraft by asking about its history, how often it's been flown, perform at least a cursory inspection, and ask to examine the log books, the broker accurately determines you aren't a real buyer.
If you were a real buyer, you would do those things before asking to fly it. Otherwise the broker will inform you he has some paperwork to do and needs to clean off his desk, because you're wasting his time.
There's a reason he hung up on you, and it wasn't because of a deficiency in his knowledge or methods.