I believe there is a such thing as peak hours. I look at it like this: the majority of BB users are from Europe. With that in mind, most of them get on around in the afternoon and evening over there, which would be morning and afternoon here in the States.
Now, if it is a game day, more people will get on to set their lineup and do whatever else they want to do. Since game days attract more users from Europe (and overall), there would be more potential users that could bid on a player. A player being sold at 2 PM EST would probably get more attention, and thus, be sold for a higher price.
Compare that to a player who is sold on a Monday night at 10 PM EST. Since most users in Europe would be asleep at that time, and maybe only users in the Americas and Asia would be online, there would be fewer potential users that would see such a player, leading to even fewer users who would bid on him.