There are a few problems with your idea, IMO.

1. When you are buying a stress-tested device, you are also going to get a device, which has been pushed to it's "limits", reducing it's life drastically. When you do that with a new device, which you own, you can be more careful, pretty much like owning a new car vs an used one.

2. People like me who have a N900, which is perfectly over-clockable may still choose not to. Because more often than not, the default works just fine. But, knowing that my device is still over-clockable if need be, because it's not been pushed too many times to the limit, is still re-assuring.

3. Which brings me to the market. I am not sure how many people would want to buy a second-hand device therefore, which may have been carelessly over-clocked to test's its limit, with no guarantees of it's life.

Just my half cents