Very sorry to hear about your sick child. I can't imagine a worse feeling. Prayers or a speeding recovery!
For selling a car, I would use your local ad bulletin, auto trader, cars.com, Craigslist, eBay. For your tractor, ad bulletin, tractorhouse.com, Craigslist, eBay.
I have had the most luck with my local ad bulletin and Craigslist, but have used the others as well. Some are free, some cost. I always start with the freebies, and only pay when I have no luck from them.