Paying large commissions is a downside to bringing the buyer and seller together and I don't think that you should pay that high of a price. That's why I built bamas.com (tho functional it's not quite launched yet) where you don't pay ANY commissions when buying a domain from a seller.
Once a buyer puts $500 in their account, they can buy $500 worth of domains and the seller gets the whole $500.
The upside is that these types of sites do bring the buyer and seller together so they should earn some fee for providing that service and trying to go behind their back to close a deal is no better than a password trading website.
