> Who would want to buy a list of people who unsubscribe?
A spammer. Before Can-Spam, spammers would always re-use and sell the address of someone who replied to a prior spam mailing, even if the reply was a threat of legal action.
If you're a spammer, there are far easier ways to get lists of email addresses of people who are much more likely to buy something. People who take the time to unsubscribe from a mailing list are incredibly unlikely to buy anything from bona fide spam. Sorry, I'm just not seeing it.
Before Can-Spam, spammers ranked e-mail addresses read by a human higher than dead addresses (no surprise). The economics of spamming dictate that a mailing list of living humans, however acquired and however unlikely to result in sales, will be spammed again and again, simply as a bet that some fool somewhere will respond as the spammer would like.
Now that we have Can-Spam and unsubscribe links must be present, the spammers have changed tactics -- instead of collecting reply addresses known to be read by humans, they collect unsubscribe addresses known to be read by humans. Old wine in new bottles.
> are incredibly unlikely to buy anything from bona fide spam.
Yes, but let's say that an unsophisticated computer user receives, not a simple spam message, but a carefully designed phishing e-mail that is known to fool .1% of its recipients. For that scenario, e-mail addresses acquired as explained above would make perfectly suitable targets.
