Once upon a time, if you wanted to buy a luxury brand item secondhand (say, a Chanel handbag) you had to have an in. There was no easy way to find one. But over the past decade, the market for secondhand luxury goods has exploded. There are now many online resellers where you can shop for used and discounted luxury items. One big problem — how can you be sure if it’s real and authentic?Some online resellers claim to have solved this problem. The…