You do not need to wait for the back cheque to clear, if it's a BANK CHEQUE, money is taken from the account and already placed on that cheque and you are entitled to the money. IF it is a personal cheque, then you need to wait for it to clear. When I purchased my car I did it via bank cheque. the seller requested BANK CHEQUE. when it's a bank cheque it's pretty fail safe exactly like how bennjamin said.

Unless the bank is in on it. You will not lose the money.