So I'm on the tightest budget right now and can't spare $250 for a Core 2 Quad. Ebay has them for as low as $100! Of course I'm looking at the new unopened ones first, however. Would this be an advisable choice?
Yeah, it depends on who you are buying the CPU from.
To be safe, you want someone with a lot of high feedback and lots of transactions. Of course there are a lot of people who sells their "old stuff" honestly from time to time and will have a low number of transactions.
I guess if it's definitly new and un opened it can't hurt but I wouldn't buy a used one on Ebay. You don't if it's been overclocked and if so how far. You don't know how used and abused it really is or if it even works. Like I said new is fine but other than that I wouldn't buy a used CPU.