its hard to tell because few years ago not that many cards had so much of a heat issue
My Diamond Monster 4MB is running great after 3 years. My Ati Rage II+ Pro is also going on 3 years... and some other no-name 1MB cards have been going on for much longer.
How much of an indication it is for a Geforce is a guess -- probably very little, as none of these cards require extra cooling.
I suppose it would depend on whether it was OCd or not. If it was poperly cooled during its lifetime or not. If you care about warranty. If you're getting a good deal or not.
It would certainly have to be fairly cheaper than a new one for me to buy it...
But no, *properly handled* cards, just like most other electrical, non-moving components, are pretty reliable and should last a long time...