It would be a stupid idea anyway. Intel lists the cheapest ivy bridge quad cpu used in the new minis at
$378. A new one mini with that cpu would run $800. If you can find one refurbished it would cost
$679. There isn't really a healthy market for the used cpus. You would probably spend more money buying used and doing that cpu upgrade than buying a new one, and that is a bad idea. If it's cheap, buy the used one for what it is and enjoy it. You couldn't get around the discrete graphics thing anyway as you'd potentially overtax the power supply with a quad cpu. A 35W ivy wouldn't be worth the money.