I'm not sure I follow? Someone bought the car and fixed it up, and is reselling it. When a car has unknown issues, that's what happens. Someone who knows how to fix it might buy it, fix it, and resell it for more. That's why it's not generally a good idea to sell a car without fixing the issues. Most people don't want the risk associated with whether it's a easy fix or not, so will only buy it at a lower price so they can't get as burned.
Also just because this guy is asking $21k on AutoTrader doesn't mean he'll get anywhere near that. AutoTrader is like the dreamer's for sale site. Prices are always way out of line with reality.
Really I think the problem is people buy/compare the ZR-1 mainly in terms of power. Every year that goes by, that comparison gets less favorable. 400hp at $20k becomes easier and easier to find elsewhere. I don't think with most older cars, people do that. They just buy it because they love that old car, and it's moderately fast. I'm not sure why the C4 doesn't command that sort of attention, or at least not from a very broad base of people. I realize it's no Ferrari or Porsche in terms of generating that sort of lust, but it's not too far removed either. Even flop cars like the BMW 8-series seem to do better in terms of selling price.