1.   Already, the Postal Service has started selling ad space on its mail trucks, collection boxes and Priority Mail envelopes.

2.   Among potential subscribers are agency media departments, media buying services, companies that sell ad space on the Internet and marketers.

3.   And newspapers were allowed to exchange ad space for things like teaching guides and pamphlets.

4.   And New York City and Colorado Springs now sell ad space on school buses.

5.   And while the Web ad glut is an economic boon to sellers of ad space, it also poses new challenges.

6.   Another warning sign is that six ad spaces were still available last week.

7.   A leader like Yahoo can sell all the ad space it wants.

8.   All kinds of ad space there.

9.   But it does come as something of a surprise that Fire Chief Chris Connealy said he is open to selling ad space on the side of his firetrucks.

10.   But they do sell ad space in specific areas under sponsorship arrangements.