I am a Computer specialist salesman at Best Buy, and I understand what you're talking about. I have a few coworkers who are like that, they'll try to tell people what they need without listening to the full question (or disregard the question).
At the same time though, you have to realize that even though you know what you're talking about, he probably deals with dozens of people a day who think they know everything and actually don't know squat. As a salesman, I admit, some days you get several people into the store who think they know everything, and when a guy who sounds like one of them comes up to you five minutes later, you automatically assume he is another one.
If I had been him, I would have said, "Oh really? I'm curious now. Do you mind if I take a look at your setup?" That way he covers two bases... one, he makes sure you know what you're talking about, and two, he lets you show off your truck. If he had done that, he would have discovered you know what you want, and he would have made you feel comfortable by showing respect for your truck/setup. Some guys just don't care though, and those are the ones I dread working with.