I understand your feelings. I don't know all the facts but if in fact he had a dual agency situation existed, it would have been best for him to disclose that to you, right?
Aside from the fact that he did not disclose, it could be that he wanted to know what you thought the property was worth so he can give the seller feedback about how to price it. If he did not have a signed listing agreement when he spoke to you, but perhaps was working on listing the property, he really would be unable to disclose that to you because he had nothing to disclose.
From the listing agent side, if you are in competition with other agents to list a property for sale, you know that you do not have the listing until the seller signs in, period. It may also be that during his conversation with the seller he said that he had spoken with you (not by name) and that based on his discussions with you (and probably other buyers) he could prove that he could bring buyers to the seller.
It would be as if someone told you that you were going to get a raise at work, you tell your husband "Hey, honey, what do you think a new kitchen would cost?", he says "more than we can afford" and you say "Well, if we could afford it what would it cost" and he says "$2,000", then you come home Friday with a nice raise that will pay it. Won't your husband wonder what you were up to?
I know it's not quite the same, but you might tell, "yes, I thought that I'd get the raise, but I wasn't sure until I saw it in writing".