I've used eBay quite a few times, and in my experience, it is reliable. Just be sure to pay attention to the seller's feedback from other members.
They are able to sell stuff for less, because a lot of the sellers get stuff wholesale, direct from manufacturers, etc. So basically, they're able to turn a profit by selling a bunch for nothing.
Other times they could just be trying to get rid of old crap. Be sure to read the item description carefully. Check specifically for terms like "used", "like new", "new and in box," "unused", "does not power on", "never opened" and things like that in the descriptions.
H-D 