"Buy now" is grossly unfair to people who have a life and cannot be on here 20 hours a day to scour for cheap players to buy now.
With 3,4,5 day selling periods, people have more of a shot of getting the player they want.
Of course, the ending bid time still looms large, because you could be at work, traveling, sleeping, doing something other than hovering over a keyboard on a fictional game with bid wars that keep going and going and going every 3 minutes.