Note also that there is fluctuation

FIFA 16 TOTY (Team of the Year) is coming. When are the player prices are their lowest? How many fifa 16 coins have you got for TOTY? Everyone knows each TOTY is the best investment time to earn coins in market, how to make coins during FIFA 16 TOTY market crash?During this time (January for TOTY), EA release special toty packs and blue cards with the best players of  Fifa 17 Coins the year (one goalkeeper, four defenders, three midfielders and three forwards).

Those are probably the best versions of a player that you can get, so a lot of packs are opened and the prices will go down massively. It is good to buy players in those moment. Also be prepared for those moments: in the weeks before TOTY, people are anticipating the drop in prices. Therefore they start to sell their players a few weeks before those events happen, which causes the player prices go down already before those events.

Note also that there is fluctuation during the release time of TOTY. For TOTY, many packs are opened while the attackers are out + while every TOTY player card is out (mostly on Sunday).

If you are buying your team back (mostly NIF cards) : I believe that the best time to buy your players back is early week. By this i mean monday or sunday. Check the prices, if they are around 30% lower than they are now then this will be a good indicator that the market is Fifa Coins at a low. This will be when everyone is panic selling before the defenders come out and the market will be flooded.
https://fifa488.wordpress.com/2016/05/30/when-a-player-comes-into-play/