dotceo, it's not just about traffic. Flippa may get more traffic, but a lot of that activity is from buyers and sellers in the sub-$1000 market.
Some larger sites sell in Flippa, but the best online businesses that come up for sale don't go to public auction. They are sold by brokers and private deals all usually subject to non-disclosure agreements etc. Brokers often have lists of buyers they contact by email.
websiteproperties is one well known place for the sale of websites. There are some more here.