I want an older dx11 card for a system i'm building for my family. Thinking of a HD5830/5850. I found a clean, tested 5830 for $40 and a very dusty untested 5830 for $25. I know it would be safer to go with the cleaner tested card, but that led me to the question of if dust can be damaging even if a card is off.
Depends if the dust has compromised the card's ability cool itself properly. Generally speaking all computer hardware will get dusty, there is just no way around this. If the dust is too much though, this may inhibit the cooling solution to properly do its job, which will result in higher temperatures both at idle and under load. If the temperatures are too high for a long time, then this could be damaging to the piece of equipment whether that is a graphics card or a processor or power supply.