Wide range of extensions
By default, magento should be having those sort options. It is a known fact that magento community version still lacks a lot of basic features. and sorting by best seller was on of them.
we have been ranking in google for a high volume search term and we have a good number of visitors and a huge catalog of about 1000 items. but we always only got about 80-90 orders at max. that left me to think that something could be wrong. and, i went to amazon to see what they have that we were lacking.
And, when i looked into their catalog, they always displayed items by their best sellers. And, this is where magento lacked and we too as an ecommerce store offering such huge variety of items missed out on the point that customers are HEAVILY INFLUENCED by what others are buying and they want to quickly buy what have been selling well.
So, we have implemented this sort order now and we used Amasty installation service and everything was set right. We are now waiting to see if this ! SINGLE CHANGE will bring in a big change in our eCommerce store. :)