Seller pays both mining fees like you. He pays the mining fee for freeze his fund on the smart contract, you pay for withdrawing it from the smart-contract.
Please note. Sometimes mining fee is very high ($10+), we can not change it
For ETH and ERC calculation the same.
Q: Do you guys take a commission fee, other than the mining fees for sending BTC?
A: No, the user pays to miners and to you
