I've bought a ton of "parts" from eBay but not a full PC. There are horror stories about systems arriving DOA, and illegal copies of the OS. Who are you going to complain to if there's a problem?
Refurbished computers from the manufacturer's sites are the most reliable, and you will get a warranty (short), and a real company to complain to or for support.
In short, I wouldn't.
I know that ebay has the cheapest deals in most things, but I'm not very experienced in that field. Is it considered a good idea to buy a computer on ebay in general?