Hi Jay,
I am Korean-American and LOVE these grapes. I believe they are sort of like concord grapes (I could be wrong though), where the flesh is easily separated from the skin. You don't eat the skin and the flesh is greenish and sweet and slightly tart and there are 2-3 seeds inside. Just be careful, my sister was visiting my parents a couple weeks back and ate a large quantity of these grapes there and suffered some intestinal discomfort.

This link I just found explains how to eat them. I couldn't really bring myself to explain how to eat them for fear of sounding slightly inappropriate--haha.
http://www.beelibaala.com/?p=891I also found this post about the YeongDong Grape Festival in Korea, with some really fun pictures. I have to get back to the motherland someday--20 years is far too long
http://another.pieceofsky.org/?p=487And I don't think I know how much a case of these usually go for--can't help you with that. But these grapes are a lot of fun to eat--hopefully you can find someone to share the case with.