That's really tough when you change your mind about 
a sale.  The trick to avoiding that is the minute 
you agree to the sale, to just put the cel away and 
not look at it again.  Pack it up ready to ship or 
something and put it out of your mind.
If you genuinly can't bare to part with it later, 
just swallow your pride and very gently tell the 
buyer that you can't do it...   You can always lie 
and tell them you misplaced the cel, but for me, I 
know it would bother my contience (sp?).

Evil Jew