aliexpress — The part of the phone is refurbished or used but mentioned and arguing as new and factory issue

Dear team, Few days ago i bought a mobile from aliexpress web site and i paid for the courier and for the customs, when i received the product and opened it i noticed that the back panel has a crack and also lot of dust in it, when i asked they are again and again they say its from factory, no factory will send out a product which has dust or crack in it, and the vendor has also agreed that they open the package for testing purpose so am not sure if the panel is changed in the testing place or where, when i enquired they are again and again arguing about the product is new and not coming to a conclusion and accepting that it has a dust and looks like its used. Am not sure if the testing according to them is using the product itself. When asked them about that they have suggested for a refund and when asked details they are behaving in a very rude manner. Please do advise on the same. http://www.aliexpress.com/about.html