For one thing, you're not buying one of the new Xbox 360 S models, but the old, notoriously unreliable, RROD-prone original models (with a 20GB hard drive). And it's refurbished. And ... it comes with a 90 day warranty.
Basically, what we're saying is don't get too attached to this $100 Xbox, because it could be out of your life at any time. Think of it as a rental for an unknown period.