This article is from the Theatre FAQ, by aku@leland.Stanford.EDU (Andrew Chia-Tso Ku) with numerous contributions by others.
You can sometimes find these people loitering around the front of the
theatre of a "hot" show just before the performance. Keep in mind that
you are taking a chance when buying a ticket from an unauthorized dealer:
even if the ticket looks authentic, it could have been purchased with a
stolen credit card and have been invalidated.