Most people assume a domain name ends in .com so if you buy a domain name with one of the other extensions (.net, .info, .org, etc.), you’ll have some extra work to get people to remember that your site has a different extension. Don’t automatically assume you should only buy domains with .com, though. Many sites have done quite well with other extensions. (Look at us!)
If your purpose in buying a trademarked term as a domain name is to try to confuse people, you’re opening yourself up to having a complaint filed against you and having to give up the domain name. Even if you’re not trying to create confusion, you’re likely to face some legal challenges by buying trademarked terms in your domain name. To be safe, you can search for U.S. trademarks at www.uspto.gov and make sure no one owns a trademark on the name you are considering.
Be extremely cautious while communicating with the owner. Even if you casually agree to buy the domain via email, the communication might be used against you in court as a legally-binding contract should you change your mind. Until you’re absolutely certain that you want to make a deal, agree to buy the domain provided that all the terms are agreeable. This will leave you an escape hatch if things go south.
You should also avoid words that have more than one spelling if your visitors are likely to be confused and mistype the name. Alternately, you can buy both versions of the name and direct visitors from the one you like less to the preferred name. While you do want a short name (see below), don’t go for something so cryptic that people have a hard time remembering it. Word of mouth is a powerful marketing tool and you want to make it easy for people to tell their friends about your site.