One thing to remember is that although there is more people bidding during high traffic times, there are also a lot more players being listed. Also many users in non-peak time zones (like the US) like to list their players during peak times, so the ratio of players for sale to bidders might actually be best during peak times. I think the average price is actually quit similar, because it is the ratio of bidders to sellers that is most important, not just the raw number of bidders.
That all being said, having only 300-400 users online definitely increases the chance that a player will slip through the cracks and go for lower than market value. But this is more rare than people make it out to be and you have to work to find these players.
IMO more important than peak time considerations are day of the week considerations. For example, a lot more people will be online Tuesday than on Friday because the former is a league night. But players listed Tues will be for sale Friday, so this gives a favorable ratio of sellers to buyers.