1- The Price: if it's too good to be true, then it's NOT true! Keep in mind that designer items maintain their value, so don't let the low price deceive you, even if it's a pre-owned item.
2- Review: Before buying, do your homework about the place or website you are buying from and be sure to search online to find out what others are saying about this place and their experience.
3- Warranty: Does this shop provide an authenticity card of its own? And what is their refund policy if proven unauthentic?
