I bet these domain auction sites like Afternic lose a lot of money
 

How can they stop someone from browsing through the names and contacting the domain owner through the whois? Afternic would lose out on commision if a sale is made off the site.

I bet 90% of their business is from domain resellers and they obviously know how to use the whois database.

The idea of most of this site is the auction...
There are sites like this one that also provide insurance that the domain holder and the buyer must exchange the money for the domain.

Yes, they defintely lose some money that way. I have been contacted directly via email from people who saw my listings on Afternic or Sedo and wanted to negotiate directly.

But I don't think it is that big of a loss overall. Most people probably do not go around the system. Remember, the transactions are initated from the Buyer. The Buyer has an incentive to use the website service because they (Afternic, Sedo) offer a secure escrow. And since the Seller pays the listing/sales fees, the it usually makes more sense for the Buyer to NOT contact the domain owner privately.

sure thats a decent point but if a site is in auction, it will probably go for more..