I don't spend more than a second or two on 'firm, non-negotiable'.  To me that implies the seller is likely a jerk about it, and I just move on unless it is a steal at the listed price.  Just my take, not saying it applies in all, or even most cases.  
In terms of asking questions, even if it's something stated in the ad, I might do that sometimes to break the ice (so to speak), or get a sense of what kind of attitude the seller has. If he's giving me short, snippy type answers to those questions, it just reaffirms he's probably a jerk, and I probably don't want to deal with him.  If, on the other hand, he uses that opportunity to talk a little more about the item that's for sale, and expand upon anything that he's said in the ad, well then I think he's probably a reasonable fellow and worth talking in detail about it.
Just how I see it.  I might be missing deals of a lifetime by looking at it this way, but if you don't want to talk, or don't want to negotiate, I don't have to waste my time.
Mike