I feel like this:
If you restrict the classified ads to supporting members only, you are charging them to pay for something they may or may not want. That alone deters me from being on this website. I am always looking at what is for sale, and many times i wanted to buy something or trade, in fact i bought a pedal off of this forum and it was a great experience.
If you had a threshold of how many posts must be made in order to see those posts then i feel more comfortable with that. However its still restricting, and may deter more people away.
With these scenarios HOW were they scammed? did they receive the money first? did they send in good faith? i mean there is a lot of things that people need to understand when posting on forums about selling or acquiring items. Some people do not see that far into the situation, others do.
Perhaps a form of authentication such as proof of address must be provided in order to use the boards for selling or buying? I suggest being organic about it, as opposed to relying on methods being used by tech.
I realize im "not that well known" here. I love this place, i would hate to leave because of poor policies in place from bad people.