Exposing fraud - deciphering email header info

I don't know if this is the correct newsgroup to be in for
help, but I have discovered a MAJOR fraud on eBay where
the crooks are utilizing Western Union to receive payment
for phony auctions. They require you to contact them by
email to set up the scam. I have enticed quite a number
of them to reply to me, whereupon I look up the email
header info to try to determine where they are. I know
that I can run the "Received from" address through
www.centralops.net/co/ and get the origin of the IP
(usually routed from a foreign location thru a more
generid IP such as AOL or Yahoo). But can any other info
in the email header be useful in determining the specific
point of origin, such as the Message ID number or the X-
Mailer info? I have one now that contains a "sub 10500"
listed after the X-Mailer program used (which is 6.0 for
Windows XP). I was told that it might be similar to a
serial number that may be traceable to the purchaser?
Would appreciate a detailed reply. Please send to my
email address at denmarkd@bellsouth.net. Thanks.