Craigslist costs $5 per ad, Facebook is free. I've had success on both if you are willing to wade through the scammers and low ballers. Autotrader also but I think my record for scammers was 20 in the first 24 hours.
Tulsa area advertisements are still free.
Edited to add, I haven't tried vehicles - just random junk, so there may be caveats.