For those interested in researching past domain sales, namebio.com is a great starting place. It has a searchable DB of public domain sales which can be filtered by extension, length, etc.

Eg. five-letter .com search: https://namebio.com/?s==ITO4gTM5ITM

Also the weekly domain sales report by Ron Jackson is another good place to get a sense of current sales: http://dnjournal.com/domainsales.htm (I believe these are included in namebio's DB, but regardless Ron adds some color commentary to the numbers)