Unless you bought it from a reseller, you don't qualify for the upgrade. I'm not sure used/refurbished hardware qualifies in the first place.
"If you recently bought a new Mac from Apple or an Apple Authorized Reseller that doesn’t include OS X Mountain Lion, you may qualify for a free upgrade from the Mac App Store."
Apple - See if you qualify for a free OS X upgrade
Solution: Buy it from the app store or "buy" it with your degraded morals, and your time, off the pirate bay. Either works
Also, pun intended with the pirate bay part.