The want-to-buy forums are part of the classifieds. I understand and appreciate the point you are making about distinguishing the want-to-buy forums from the for-sale forums, but they are all part of the classifieds. (For clarity's sake.)
In these sorts of discussions (and many other types, to be honest), I don't much care for terms like "perhaps" and "a bit more likely". What is certain is that only supporting members can list things for sale, and both supporting and non-supporting members alike can buy those things that are listed for sale. Limiting sale listings to supporting members is both a privilege of supporting membership and a measure of security. Nobody seems to have a problem with this arrangement.
I see no reason to change that. What I mean, specifically regarding the want-to-buy ads, is that non-supporting members should not be allowed to sell on TB, and therefore the want-to-buy forums should not be available to be used as a dodge around that policy. Anyone should be able to list something in the want-to-buy forums, just like anyone can buy things that are listed by supporting members in the for-sale forums. But non-supporting members should not be allowed the privilege of selling items to want-to-buyers, just like they are not allowed to post things for sale on TB.
So if TB establishes a rule that anyone can post in a want-to-buy forum, but the transaction can only be completed in the for-sale forums in order to be sanctioned by TalkBass, the status and privilege would be preserved. With such a rule, anyone wishing to respond to a want-to-buy could discuss things with the potential buyer in the WTB thread, but ultimately that person would have to list the item in the for-sale section before the potential buyer could buy it.
Hopefully there would be a link in the WTB thread to the FS listing of the same item, but it's not really necessary. And of course, people could still skirt the rule just like they sometimes skirt the rule about no-fee PayPal requests, but the protection would be in place for those who follow the rules.