When I'm at a con, I expect prices to be a little high. I look at it as I'm supporting the continued presence of dealers and comic conventions in general. It only takes one walkthrough to realize which dealers are sellers motivated to move product, and which would rather sell less books at higher markup. After a few years, I might even start to remember which are which!. I generally don't make offers, but gladly accept them with a sincere thank you! That said, I am that guy (old school), with his handwritten buy list on a piece of paper, and current GPA written down across a spread of grades. I generally like to make a little weekend vacation of it, so I understand that they have expenses like gas, hotel rooms and meals.
Also some con dealers don't overprice, they overgrade the old-fashioned way! I prefer slightly more accurate grading with maybe a 10-15% markup. If I get a deal on a stack, BONUS!