I bought a couple of open-box S939 mobos from the Egg, a 939dual-sata2 and 939dual-vista. Boot-tested them when I got them, they worked. Never installed them into working systems.
Well, a friend needed a replacement 939 board, so I gave him a 939dual-sata2. Turned out, after running for 10 minutes, the northbridge gets too hot to touch. A different friend is currently tested the system, with a fan on the northbridge. Seems to work so far.
But that just goes to show you that there might be non-obvious problems with open-box products.
On the whole, though, I've been sucessful, although recently I opted to purchase a new K9A2 Platinum, rather than risk the open-box, because I wanted the warranty.