The University of California, Berkeley, has identified a black market for its Class Pass, a transit permit that allows students free access to AC Transit buses during the school year.

A recent search of Craigslist in the San Francisco Bay Area found half-a-dozen Class Passes for sale between $40 and $70. Sellers often are Cal students who don’t ride AC Transit buses.

The Class Pass is a sticker that affixes to a Cal student’s ID card. The program, which was started nearly 10 years, is funded through a $58.50-per-semester transportation fee that all students must pay.

Buyers are often former Cal students who still have their ID cards and are frequent users of AC Transit’s bus system. Over an eight-month period, black market buyers of the Class Pass could save more than $500, based on the cost of purchasing eight $70 monthly passes ($560 total) from AC Transit.

Some students justify the fraudulent sale of their Class Pass, arguing that they are forced to pay a transportation fee for a bus service that they don’t use. Selling the pass, they say, is their way of recouping their fee.

There’s also growing concern about counterfeit Class Pass stickers. Students who lose their passes are charged $60 a replacement fee. Rather than pay the fee, they borrow another student’s pass and use color copiers to reproduce the sticker.