Depends on exactly what it is, what condition it is in, and whether it fits you.

You can find some beautiful bikes, that are much better than what you could buy a similar bike new for, on CL. You can also find a lot of dogs. Most of the battle is in knowing what is worth it, and what isn't.

Any link to the bike in question? I'd be inclined to say the Specialized is probably the better buy, but that would be a guess without seeing it.