This is taken from their FAQ:
How is the transaction fee determined?
In the gold-based auction house, a 15% transaction fee will be deducted from the final sale price of a successful auction.
This is not an explanation. It is just stating what WILL be done. How is DEDUCTING 15% from a gold sale, considered a transaction fee?
A charge an intermediary, such as a broker-dealer or a bank, assesses for assisting in the sale or purchase of a security or money.
This is fake currency we're talking about here, gold that blizzard makes, not real money.
Overall, this is very discouraging & makes zero sense.