I like the idea of RJ, don't like the reality, currently.
I also don't see how a 'general interest' auto site can be successful with online classifieds.
If you're small you have to be specific. I sold a bunch on z31.com when I had a Z car to part out. Great market for that specific car. A turbo crossmember sold for $90 in a matter of hours IIRC. It would elicit shrugs here and on CL, probably.
There are probably 100's of good item/interest specific online classifieds that work great.
Practical Machinist is another that I use. Great community.
On the other hand ebay and CL are large enough and have millions of daily viewers and easy searching.
Hell, you can google what you're looking for and find it also via CL and ebay (and many other sources).
No real solutions here.
I'd say a post count of 25 and have a classified free for all...with the common caveats: No commercial ads for example. Maybe have a rolling delete ala CL where every post self-deletes after 30? days.
I guess the above wouldn't help GRM financially, but IMHO you have to be very specific or very big for online classifieds to be a moneymaker. (and in the case of the very specific it's probably not a moneymaker either, just people who love X enough to religiously promote it).
-James