There is a healthy, supply-and-demand driven market for tickets outside Cameron (and pretty much every other venue I've seen a game in), both fans with extras and scalpers. For an early season or not-quite-marquee game, there are usually lots of tickets available for face or below. Just wear your Duke stuff! And having a cute kid in tow always helps.
The scalpers work Cameron right under the noses of the Duke police, and have been forever. Some high tech operations, too - buyers out on 751 or Science Dr with walkie-talkies to the sellers at Cameron with runners (and once a roller-blader) in between. I can only surmise the University turns a mostly blind eye to it, so long as the transactions themselves are reasonably discreet. Frankly, you can scalp yourself in for the entire season for less than it takes to get season tickets.