That's very easy to say, but the reality is more complex. Much of the attraction of Craigslist is the free posting. Not all the groups are commercial in nature

That's very easy to say, but the reality is more complex. Much of the attraction of Craigslist is the free posting. Not all the groups are commercial in nature, many are just discussion. Also, the Craigslist founders think of the site as a public service. No convenient and reliable micropayments system yet exists. Cost per transaction on credit cards is far too high. Online games use private currencies; you buy a big chunk of currency and you draw on it for in-game transactions. That could work, and it kills spam dead, but the big upfront cost would make many users hesitate before posting. It's not like a game, where you can expect to play for hours at a time. I bet most Craigslist users post perhaps once or twice a year. Extra screens for purchasing would also complicate the UI. Another deterrent to posting. It's not clear to me that you could set a price which deters spammers but allows the good postings through. The economics are different from email spam. A spammer can flood a Craigslist group with just a few hundred messages, and then Craigslist will do the work of distributing it to millions. Furthermore some Craigslist spammers are real local businesses that just post too frequently, or in improper places. Like a local car dealership flooding the used cars group. The dealership's expected return might be a few thousand dollars, and they may be perfectly willing to spend $20. Maybe payments are part of the solution, but I don't think they can be the whole solution; at least not if Craigslist wants to be like it has been in the past.