I LOVE yellow ZR-1s and that looks like a really nice one! Yellow one is in my top 3 favorite for a Z, which includes Admiral Blue and a Black/red combo. One of these days I'll have one of those three to go with my Red 90. By the time I'm ready it'll likely be with high miles and need some work but I don't care; will be worth it.
Was in Chicago last weekend for Easter, I have a relative that lives literally 2 minutes from Corvette Mike Midwest. He has a Yellow 91 ZR-1 for sale there right now also. My wife would kill me if I went and looked at either but I had to look at the details to determine they weren't the same car.
I personally think that the $21k is very reasonable for this car with its history. For comparison Corvette Mike has his listed at $29k, which I think is on the high side but not super unreasonable for what he's got.
Here's the link for anybody curious