Just check the trademark for the name you are interested in here
http://www.uspto.gov/ , Also check to see if the other extensions of the name you are interested in are developed, say if you pick up the .us of a name, all other extensions are regged, Do not develope your website in the exact same niche, this could cause a UDRP.
Hold onto your names a while before selling, Unless you pick up a name that has a very overture, Alot of back links and ect, Then you have the option to sell right away if the name has the above, You are far better off developing your names into websites, getting them indexed with search engines and submitting them to website directories, They will get google page rank over time, advertise and promote your names, After a year or so, you will know what names are going to make it, and those wont make it, You are going to make a much larger profit on a name, Based on one year of revenue,traffic stats, Than you will by just quick flipping a name, and making 3 or 4 dollars. Just my 2 cents, Best of luck to you.