Volkan - I hope this gets to you before it's too late!!! I am in the UK - I have recently tried selling a plane through the BMFA web-site and have had a very similar situation arise. Someone from overseas with a client owing them money in the UK - the client sends me a bankers cheque for way over the amount and then they want me to wire transfer (western union) the balance to an address in Nigeria.
I was suspicious so have sniffed round the web and found this>>
http://www.fact-index.com/n/ni/niger...fer_fraud.html
There are dozens of refernces to the Nigerian(or various other countries in Far East etc) scam on the web.
DO NOT WIRE TRANSFER AND FUNDS FORM YOUR ACCOUNT - even if the apparently good bankers draft/cheque etc clears... it will eventually get rejected and the funds withdrawn from your account after you have wire transferred out.