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Unfortunately you can faced with situations on Upwork(oDesk) when customers do not pay at all. There are some regulation rules for such situations, but anyway it is better to choose right people from the first attempt.
The most important thing which you must do before starting any relationship with a customer - read freelances' feedback. You can learn more about a customer.
But anyway, if you have positive expectations then as a rule even bad customers will be gold for you.