Buying:
When fewer people are on line - usually in the evening there as few as 400 people
Doesn't matter which day of the week
Selling:
Game days - all day but peak hours are usally about noon until 5 PM or so
Selling before paying salary:
Not any more - now you must pay salary at least once
unless you are getting cheap talent just for scrimmage and you fire them after
Best time of season to buy:
Midseason when most rosters have been filled
Best time of season to sell:
Beginning and just before playoffs
feel free to correct my mistakes