Good to hear you think this is the place to sell a Zed. For anyone in Australia I would like to think that this is the best place to buy and sell a zed, just Google 'Datsun 240z' or 'Datsun 260z' from Google.com.au and you'll see why.
I don't want to lock it down. I would imagine it would be very frustrating to have to login in just to view the for sale section. I actually think forcing people to register could discourage people from registering as oppose to encourage.
Yes I know this happens and I see the same trend looking at our Web stats data. I'm actively looking for ways to encourage people to join our website rather than just browse and lurk for ages. There is definitely a snowball effect here, in that the more people who join the more want to join.
We have been certainly growing in terms of member numbers very rapidly over the past year when compared with the first couple of years we were around which is positive.
So at the end of the day I don't see a reason to change anything, other than to make the site easier to use, less cluttered and more useful to the members.