I can (somehow) understand why there is a 5% tax on selling a player, but I feel the tax should clearly be reworked.
First, packed players should be tax free if they are sold with 0 games played. That's not trading, you got them in a pack.
Secondly and more importantly, taxes should decrease and/or be capped as it completely makes trying very expensive players too costly.
If you were to buy a player at 1 mio and want to sell it 10 or 20 games later, you'll lose 50k. If you're in a hurrry to sell for whatever reason (e.g. you don't have too much time to play AND want to try another player who just would fit your team), you'll lose more to sell him quickly. If the market has gone down, as it has done quite often this year (not to say continuously), you'll lose also more on top of the 50k tax.
I tried Ronaldinho 91 the other day and decided he's too big an investment for my team (and that I was somehow missing Neymar too), and I just had to sell him and take a 150k loss by doing so.
It's not normal. While if you buy a player at 30k it's kind of ok to lose 1.5k as a tax when selling him, but the tax should be capped at say 30k. OR become 4% at 500k, 3% at 1 mio, 2% above 2 mio, and like 1% above 3 mio.