• They send or give you a check or money order for more than the purchase price and ask you to send the extra to someone who will take care of shipping. But there's no reason why they can't send that person the money directly.
• They say that a check or money order payment will come from someone who owes them money and tell you to deduct your share and send them the rest. Maybe they're in a foreign country and because of currency differences it's difficult to pay you directly. But it's easy to transfer money electronically from anywhere - there's no reason to have someone else send you payment.
• They claim they sent the wrong amount "by mistake" and ask you to return the excess.
• Legitimate buyers will be happy to send the exact amount you're owed.