Day trading is a tough thing to do in BB. Mostly because of the high 20 or 21 % fee you have to pay if you sell the player immediately. Plus, BB's new rules made it impossible for players to be sold until you pay at least 1 week of salary.
Let's say you buy a player who costs $100k and his salary is $10k a week. You have to sell him for $135k just to break even. If you want to make a good profit, you have to sell him for much more. And that is not easy in the ever declining transfer market. You need to be very lucky.
On the bidding part, I think 3 minutes before the deadline is the best time to bid. Bidding 10 second before the end will add 3 more minutes on the clock, meaning more time for other managers to spot this player.