There are types of domain extensions to consider when you are getting yourself into the business of buying and selling domain names. The most desired are .com, .org, and .net, so if you come up with a great site name that is available with these extensions, consider yourself lucky. These extensions are available world-wide, so anyone anywhere can access the web site. Some of the other extensions are country-specific, and if you plan to offer or sell a service in a foreign country, they will serve you well. There are services where you can get cheap domain name registration, and for the most part, the services for the cheap ones are the same as for more expensive domain name registration sites.
Some people make good livings buying and selling domain names, and if you happen to enjoy coming up with short, catchy names that are sticky, go for it. It costs very little to get into this business, and it's also fun to make names up for targeted markets, whether they have a narrow audience or a wide one. You can also buy expired domain names, and try selling them yourself.
