Your recourse is against the seller who apparently defrauded you. As the poster above noted, this forum doesn't condone threads talking about manipulating IMEI numbers and if a moderator sees your post, no doubt the thread will be closed, etc, etc.
This is the wrong forum to talk about IMEI changes. As you've been a member since 2011 you should know that. Or would if you read even a handful of forum threads. Try to recover from the seller e.g. make a credit card dispute, Paypal claim, stop payment on your cheque, police report, etc as appropriate. If you are unable to recover from the seller, you may end up selling the phone for parts on Ebay or something.