I tried to find some answers, but didn’t quickly see much info on bulletin board spammer income. Plenty of info on email spammers, but that’s a different kind of operation.
Payment is probably similar, I.e., they get a commission on sales (assuming there’s something in the links they are posting that allows them to be identified as the referral source). Anyways, on an unrelated subject you may want to check out
https://twelve-inches-guaranted.com.
“Spammers
Spammers send out millions of messages on behalf of online merchants who want to sell a product. If a spam recipient buys something, the spammer gets a percentage of the sale. For pharmaceuticals the commission can be as high as 50%, and research has shown that the response rate can be rather high. A good example is "penis related spam" which has a 5% click rate, meaning that 5% of the recipients actually open the spam mail and click on the link in the mail.
This means that spammers can make a massive amount of money. In July 2007, a retired spammer told PC World that at the peak of his power he pulled in $10,000 to $15,000 a week sending e-mails that promoted pills, porn and casinos”