I know this is old, but I still think it's an important question to answer. Especially nowadays.
A lot items these days are so expensive that it would take several years to acquire them. For example, trying to get a stamp avatar would take FOREVER. So if you have a hard time making NP or just don't want to spend that much time earning them, buying neopoints is a decent option.
So many of the sites selling NP these days are illegitimate though . Personally I only buy neopoints here, because they at least seem to demonstrate they care about my account.