A trade mark needs not to be registered anywhere.
Trademark by usage do exist and recognized by the USPTO (United States Patent and Trademark Office).
Here You find it clearly explained:
http://www.uspto.gov/web/offices/tac/doc/basic/register.htm
Domain trade is more than posting around: It is mostly research.
The best way to go into domain trading, is to spend upon an experienced lawyer and have him/her explain why they give a certain advice and where they got the information.
The easiest way to go into domain trading, is to assume that most people who bought, sold, or appraised some domains are experienced enough to advise in the field. This is, of course, pure gambling: The only standards applicable in the domain industry are those of Local Common Laws, Trademark Legislation (international treaties - i.e. Madrid Protocol etc.), WIPO (wipo.org), ICANN.org, IANA.org, W3.org and more a few of those.
Any other advice concerning domains are opinions, based upon individual in-house information that is usually not complete with private (business, financial and other) data about seller and candidate-buyer.
Past sales are no reference as to experience, but about willingly buyers.
Overpricing and underpricing are opinions, usually from jealous people.
Choose a domain, do a few search engine lookups to see if there are related domains, make up Your mind if You want end-user or reseller price, then set a price, promote, and follow-up.
The ugliest girl gets married, too - If she's good in cooking and the rest.