Used Mac Pro
Well, that would depend on who owned it and what they used it for. If they were just thinking about graphic design but didn't actually use it for that and ended up using it as a normal computer, I wouldn't offer more than $2900.
If the previous owner was a serious artist that used it for graphic design for the majority of the time they owned it, then perhaps no more than $2500.
Are the normal accessories included as well or do you have to provide your own keyboard/mouse/trackpad? Do you want Windows 7 to remain on the drive?