Con men/women are good at what they do. The more convincing they are, the better they fool you.
Also, please understand the meaning of a check "clearing" the bank. You may deposit a check and the teller will tell you it will be available in "x" days. That does not mean the funds have been collected and not subject to some kind of return. It only means that by government regulation a bank can only "hold" for a set number of days.
If you simply must make the sale, do as TexTrainer suggests. Ask your bank to send the check for collection and only ship when you have received funds from your bank.
Search these threads for "Nigerian" and you will become more educated about the various spins of this scenario that are out there.
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I try to take one day at a time -- but sometimes several days attack me at once.