If I'm selling directly to a friend, I usually 
price a cel at or around what I paid.  Sometimes 
under, if I know for a damn fact that I paid way 
too much to start with -- since I have an 
obsessive personality, this happens more often 
than I'd like, heh.

But for sale to the general public?  Heck, I say 
you should just toss that sucker on eBay and let 
the forces of popularity determine its price.  I 
can't see how anyone could fault you for that!