you use in commerce are very valuable assets that you need to protect.
A domain name is sometimes used as an Online Name identifying a
registered user of an i-mall or auction. However, keep in mind that
a domain name in itself does not automatically confer a trademark
on the domain name. The trademark has to be used in commerce to
qualify as common-law trademark (TM). Protection of your domain
name is even more important if it does not end with dot-com, which
suggests that someone else registered the same name ahead of you.
(Typically, dot-net and dot-org are registered when the dot-com
has already been registered.)
if you are a registered user on an auction site or provide consulting
services on sites like keen.com, you need to protect your Online
Name, which is your online identity. You don't want others to be
using your identity on another site.
an Online Name in commerce gives you the right to register it as
information on trademarks, see TM-it.com