Goes to show you, that even if you fight like a mad dog, you still may be leaving money on the table.
Yep, I really have no way of knowing whether or not customers are bluffing. And buyers know that which is why they ask for discounts anyways. And when they do, I almost always give them at least something.
Some of the accounts I work with are too big to take the risk as long as I still have money left I can give them, and I almost always do. The trick is knowing when to say enough's enough and draw a line in the sand before I'm stuck with a multi million dollar account that consistently loses money.