I'd say it's all down to how much you like the cel
vs. how much you like the person.  *grin*  I've
given up more than a few cels and/or sketches to
friends who wanted them more than I did, and I
know that some of them have done the same for me.

But if you really like the cel, don't have any
major emotional attachment to the person, and
don't need the money, then you shouldn't feel even
a little bit guilty about keeping that sucker.  :)