I'm going to keep this as short as possible.
As an experienced merchant I STILL hesitate when making first offers. Some items are commonly traded and their value established. I don't hesitate when someone asks me hwo many linen for metal. But I do when Someon asks how many fish for HQ TPP. Don't get offended if someone lowballs you. Expect it. It's the nature of a barter economy.
Sometimes you wont reach and agreement. Sometimes you know you can milk the guy for a few extra pieces. But don't be offended, don't laugh, and don't insult when you get an offer. The guy who makes the first offer on uncommonly traded items is going out on a limb. He might be paying too much. He might not be paying enough.
Always make a counter-offer even if you're willing to pay the rate proposed. If they say no then you can always go back and accept.