Don't be afraid to pepper sellers with an annoying number of queries. How do they know the item is authentic? Where did they get it from? When did they get it? What did they pay for it? How old is it? Has it been restored? The answers are helpful, of course, but even more crucial is how the vendor reacts. "This is authenticating the seller," says Dowling, "and the only way you can do that is ask questions. If someone isn't happy to answer those questions, walk out." Good sellers won't mind answering any question – and they won't pretend to have all the answers. "So if you've asked 20 questions and he answers 15, but to the rest says 'I'm not sure, but I'll find out for you,' then all of a sudden you've got an honest gentleman or lady in front of you.