The dent is significant enough to have caused the frame to tweak, so have it checked. It can also be a point of failure in the future. Not likely, but possible.
The frame is the single most important component on the bike. In its present condition, I'd say the value of the frame has been reduced to zero. If the price is reasonable for a parts bike, then its a good deal coz it will get you riding.
If its a really good deal I would just buy it and not worry about it much. If it breaks it will simply be an opoortunity to change rides with a new frame swapping everything over. I would agree with the previous post that the frame price has been reduced significantly.
Kona frames are overbuilt...the Coiler is designed to take a beating. I doubt that dent will ever cause a failure since there are no creases or stress risers in the area. As long as it hasn't affected the overall alignment of the frame, I'd say no big deal.
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