1. Fee that the certain platform might charge
2. Difference between buying and selling price
3. Fee for transferring to your bank account as well
You can do certain things to avoid some of the fee for example:
Selling them p2p
Finding other relevant exchanges
Find other wallets as well
You can find exchanges like kraken as well but it's a trading platform mind it exchange is different than a wallet so don't hold your coins for too long.