I will be relaunching my new site within a few weeks, and I need posters in my new free classified ads section. Most sites in my niche charge for a classified ad post, so I have a clear advantage there. However traffic will be low at the relaunch, AND how do you convince users that a free posting is worth their time when there are no or very little ads on the site already? An idea that crosses my mind is to contact classified ad posters on other sites, but that can be spammy, and many times you can only contact the seller through the website. I'm worried about "sabotage" for my own site if I spam other sites. Maybe i'm overly worried for nothing? Another idea is craigslist ads. Posting a "Wanted: Classic Cars" ad in craigslist, then having my url in the ad (I know it cannot be hyperlinked per craigslist TOS). Would I still be causing trouble with craigslist doing this? I truly wish it was summer, and not zero degrees. There are car shows every weekend in my area in the summer, and that would be a perfect place to hand out flyers to people selling their cars.
You seem to have it pretty well thought out. You could contact people selling cars directly by phone or e-mail if they have an ad running somewhere. Ask permission to put an ad for them for free. It would be a bit of work. If you chose high priced, hard-to-sell cars, they wouldn't want to take the ad down right away.