How do you negotiate for stuff that have momentary value?

First, when you trade, you are impatient. Your "need" for the item and your fear that you might not get the item drives you on and makes it difficult for you to negotiate.

Second, you are afraid of the consequences that may arise from your arguing, most notably, in your case, awkwardness and embarrassment from potentially losing the argument. You are too wary of how the storekeeper or your boss would think of you.

You are not afraid of negotiating with your teacher because you've got nothing to lose there. Teachers nowadays are basically there to serve the student, even if unintended, and if unsuccessful, you can complain and whine about the teacher with little consequence.

Well, at least that was how I saw it.