If i understand your question correctly, then you are asking if its possible to change e-mail address in Webzen account?
-This is not possible as far as i know, but you could try to write a ticket to Webzen from that account to get it changed, or you could create your own email address, and then tell the seller to send a ticket to Webzen saying he wants the old email changed to the email which you created (I dont know if this would work, but you can always try)
Im not really sure what you mean by your other questions, but no its not possible to cancel a payment on Paypal, and yea anyone could just open a case on paypal to try get their money back, which is why i dont really like to use it
Hope this helps a little