I have the board, and it should have green heatsink on the northbridge.
I personally don't take boards with put down heatsinks (untested), only if price is really low and risk is worth. I've took some of them worked, the most suprised me Asus P4P800 Deluxe, which even had down all jumpers, and capacitors was buldged, also without NB heatsink, so I was expecting it will be dead (took it for about 2$), and after I've put jumpers on the board, it posted, was suprised. You clearly didnt have such luck, I hope, you didn't pay too much.
The board doesn't have to be damaged from heatsink removal. It's like that: when heatsink is down, someone used it as donor of parts, and it was dead before. That's why can be something missing. He let the board, in case, he will need something other, or... he will meet someone, who have chance to repair the board. So, northbridge don't have to be damaged, because heatsink is missing, board can have different problem, and that's why heatsink was removed and used on other board or some card.