We are implementing an addendum to the sales tax that is based solely on the number of trades a team has made in the past 14 weeks. If you have only made a couple of trades, you will be charged the normal sales tax; if you have traded a lot recently, you should expect taxes that may be several times as high as the regular sales tax.
The fees and taxes for a sale are determined by two factors: the amount of time that the player has been on your team, and the total number of sales your team has made in the previous 14 weeks. The fees are lowest when you have owned the player for a long time, and when you have made few sales in the preceeding weeks. Each of your players will have the currently-applicable fee shown on their individual player page.