Great questions. I think Kats answered the first. When we're asking questions and recommending web sites or specific links, it's not spam.
However, if we are sending each other unsolicited pm's or e-mails saying visit
www.spendmoneyhere.com, now that's spam:
http://www.bdsmlibrary.com/forums/showthread.php?t=1687
Yes, they do. Many sites sell something. For example, Literotica has lots of free areas, but you can also link to its toy/book/clothing store.
http://www.literotica.com/adult_store/
Yet you don't have to pay money to read stories, previews, etc. The same goes for this site.
If a member of this forum responds to a request for information by providing a link to their own site or another and that site happens to sell things, it doesn't violate any spam policy.
Now, if a member ran from thread to thread and went wild, posting something like "You must see this site ... www.spendmoneyhere.com", yet didn't add anything to the value of the thread, I may not call it spam, but I'd request a moderator smack down.