I don't think of it that way. They're not really taking 15% of what you make, it's more like a traditional auction. If I take my faberge egg to an auctioneer, they don't expect me to pay them commission, they just take it from the person who buys the egg. It's called a buyer's premium.
I agree that it cuts into being able to profit on the marketplace somewhat, /but it's not there for you to profit from/. It's there for Valve.
That being said, it does look mighty appealing to buy up some keys and sell them for 20% profit. (Buy at 1.40, sell at 1.70, fees put it to the buyer at 1.96), but the volume isn't going to be there to sustain that kind of spread.