A. Famous marketplaces that you can see domain name prices;
. Sedo.com
. BuyDomains.com
. AfterNic.com

B. Why you should have .com domain;
There are so many country code top-level domains(ccTLD) and generic top-level domains(gTLD). You can see the list of internet top-level domains at http://en.wikipedia.org/wiki/List_of_Internet_top-level_domains . The internet surfers automatically type “com”, if they do not remember the extension(TLD). If you advertise your web site with any other domain, the web site with .com will get more visitors automatically. If you have .com domain,there is no need to hurry to get the other domains. But if you do not, even though you have many other domains, you should have .com domain for business.