Great question. I purchased a coin of Kijiji about a year ago and after receiving it I felt that it wasn't real silver. I ran a few tests and results were mixed. I contacted the seller and he returned my money and paid for the shipping of the coin back to him.
It is a low margin of profit for these dealers and if someone gets ripped off or feels like they got ripped off by a certain dealer it spells doom for that dealer. So I can not speak for everyone's experience but in my experience if they are reputable dealer they will always take it back and refund the money. Hope this helps. Thanks for the question.