To find out details about establishing your own business, which in essence is what you are doing,you may want to start at the local level first to see what the requirements are.
If you are in the U.S., one place to start is the U.S.'s Small Business Administration(SBA.gov).
Here is a link to a section of that site talking about what is required for the filing of business names, listed by state. Not all states require the filing of a business name but if yours DOES, there should be a link you can follow to do a search to see if the name is already in use:
Here is an article from the Wall Street Journal that references a site you can search for names that companies have registered on a federal level:
Keep in mind that the Internet is still a sort of 'Wild West' for many small hobbyists - meaning, people may be selling items under a 'name' that they are using as their 'company' but haven't bothered to follow the legal requirements to make their 'company' legitimate in the eyes of the government. Finding someone online with the name you want to use doesn't necessarily mean they are recognized as a business by the government.
That being said, you may also want to do a domain name search to see if anyone has registered under the name you want to use. You could use http://www.godaddy.com/domains/search.aspx or any other registrar to check that out. If nothing is found, that may also be a starting place for you (to purchase that name) so that no one grabs it - you never know when you may want to start your OWN website using your name of choice and you don't want to be disappointed to find someone already has your '.com' name (even if they 'haven't' registered that name with their government).
Hope that helps.