the prices are pretty fluctuant on BB, maybe because there are to less people who searching at the same time after the same players ;) So i would say with a bit patient you could buy on everyphase on the season.
New trainees are maybe best at the beginning of the season, and in the season break you could buy a bit cheaper(thats for me the time beginning with the transferdeadline for the playoffs - maybe the prices are a bit higher short before the deadline)
The same i would say for selling, to put someone on the list is priceless, so take an high first bid and sell him to an 'idiot' who can't wait ;)