But since the success rate for SPAM is so low, it only takes a miniscule per-post charge to make it uneconomical. Also, the requirement for payment could be structured to make it much harder for spammers to make arbitrary new identities.
Even if spam never saw a financial return, there would still be spam.
A lot of spam is purchased as a 'marketing service' by fools who _think_ spam works, who make a payment to a third party to spam on their behalf.
As the failure happens after the payment the effectiveness or lack thereof only modifies the decisions of repeat offenders - and even then only those who actually learn from their mistakes.
Of course spam will stop once the world runs out of fools who think they can get rich quick... any day now, really...
The population of fools who think they can get rich quick is related to the incidence of fools who do get rich quick. If no one gets rich spamming Craigslist and everyone instead gets poorer because it's too expensive, then no one will spam Craigslist.
Combine the per-post micropayment with the forfeiture of your account balance one if you are caught spamming and I think this would be an effective deterrent. There will be fools who will still spam. They will just do it elsewhere.
It's like the two guys in the woods confronted by a bear. One ties his running shows and the other asks if he could really outrun the bear. The first guy says: "I don't have to outrun the bear, just you!"
