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Just really basic saying can change the whole scenario for i.e;
Welcome to the store
I would be happy to assist you
Please let me know if you have any question about the product we have for you
Try in such great manner why customer is not pleased about the product. Listen carefully about the product. Understand and give apology for the inconvience and resolve the issue.
Also, make a note to let your manager know about this situation so it won't happend for the next customer!
Give your best to convince them, try to make them understand about your product, rest leave it on them and Always try to make your customers comfortable.