3. The hidden cost
One of the things that attracts some people to the idea of advertising via Usenet is that it costs so little to do it. You pay $20 per month for an Internet account, and you can post literally millions of advertisements at no additional cost.
But there is a cost. Each message you post takes up disk space on each site around the world where it lands. People don't much mind paying for disk space to hold Usenet discussions since they know that people like taking part in those discussions and since they know that it's sort of like mutual backscratching. Everyone benefits from interesting, informative, or amusing discussions, so no one really minds paying for the space.