Most people are very proprietary about their nicknames.
In the past, many became quite upset when a newcomer to the board usurped their nickname or, worse, deliberately impersonated them. Fortunately, the new handle registration system has pretty much solved that problem by allowing each user to register their handle so that no one else can use it. Choosing a handle in the first place can be tricky, and since you need a handle before you can register to post, I decided to talk about this first. There's no listing that tells you which handles are already taken. If you'll just try out a name that interests you, and you attempt to register a handle that is in use, you'll get a message saying it's not available.