They may send you a check for more than the item and ask you to send the difference to them. Never do that.
Unless you get cash in your hand, you have to let any check you receive fully clear the banking system. Your bank will give you access to the funds in 3-5 days, but it can take as much as 6-8 weeks before a fraudulent check is returned to you and then your bank will debit your account.
My recommendation is never to accept a check from someone overseas, especially from Africa or Asia, without waiting 8 weeks before you let anyone have the merchandize.
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