My rule of thumb is 30% less than the asking price is a lowball. I routinely offer between 10%-20% off just to wheel and deal a little but if you have something priced appropriately I'll sometimes just take the price if I really want it.
Case in point for what I feel was a lowball:
I recently offered an item for sale for $70 and dropped it to $60 after a few days. I got a PM almost immediately asking if I would take $40....