Well, I for one would be quite happy to pay
shipping in order to reunite a cel and its sketch.
 :)  It's an understandable mistake -- just sorry
you have to go through the trouble now of figuring
out how to resolve the situation!

Keep in mind that there's a lot of trading among
the cel community, so even if the people you
traded with 5 years ago aren't in the hobby any
more, others here might have some of their cels. 
I'd go with the previous posters' suggestions and
make a page of scans, then ask people to send a
cel scan and sequence number if they think they
have a match.

It'll all work out in the end.  *hug!*