What are some of the most usable websites? Why?

http://Flippa.com does a great job simplifying the "buy a whole business on the web" process.

There are 2 buttons, "buy website" and "sell website" that split traffic accordingly and make things happen.

They also did a usability test with https://UsabilityHub.com and made the results public, which you can see here:

https://usabilityhub.com/tests/c942c855fa6/results/fbee3843a15f