Spiritofthewater, I knew I'd seen that Nightfall sketch before. It's on Marty Kuhn's website in his "Nightfall Art" section (http://www.rwhirled.com/nightart-warp.htm). He doesn't own it (obviously, or the seller would have known who it was). Here's what he says about it:
"Wendy Pini, 1995. A large grease marker/crayon of sorts was used to draw this fine portrait at the 1995 Chicago ComiCon. [GIF, B&W, about 15k"
Marty says that he hopes for a better computer file of this pic, because the one he has is not too good.