Have a scan handy.  Then post a listing of 
dougas/sketches you have with a description and 
sequence number.  Have the person who has the cel 
also email you a scan of their cel, and if 
everyone agrees it is the same, and they are 
willing to pay shipping, then we have a reunion.

Unless you originally advertized the cel as 
coming with a sketch, then I don't believe you 
are responsible for the shipping costs, and I 
agree with others who have said they'd be willing 
to pay for shipping to reunite cel with sketch.

Of course, all of this assumes you have the time 
to process all of this backing and forthing... :)