I never buy players in the first few days of a season, gets worse every season. That is the best time to sell players.
I generally stock up on young 18yos in the final days of the previous season, sell the *** players at the start of the season then restock as the season progresses.
The only players I buy at the start of a new season are 22 or 23yo scout players that are close to becoming 7*s.