I wonder how much those spammers make per bogus listing. If it's more than what legit advertisers make per listing then charging per listing will only drive out the good guys or, at best, do nothing. If it's not, charge somewhere in the middle and, voila, problem solved.

I guess the reality is, you can only have a spamerless site when all your users are more lucrative than spammers.

Anyway, even if it would result in higher profits, CL would not be interested. Because they're not interested in driving up their profits.