While not tractor related, I sold a shrink wrap machine through a 3rd party site and all they did was give me the prospective buyers contact info and left it up to me to make the deal. They asked for 10% of the sale price and trusted me to pay them.
I arranged payment in advance with the buyer, who was agreeable and I sent the 3rd party folks their cut. Everyone was satisfied with the deal.
GLWS.
Tractor house is basically just a classified site. I believe the first two are as well. As far as I know they just connect buyers and sellers, they don't handle the actual transaction. All seem to be about 95% dealer ads. I bought my tractor after finding it on Tractor House. Went to the dealer to check it out and told the salesman how I came across it just so he could know his advertising was effective, but he didn't seem to care. I look on Auction Time a lot and have bid on a few things, but never won. I can't recall ever seeing something listed by a private party there.
I have never sold online, but I have made large purchases. I bought my M7040 sight-unseen from TractorHouse. For me, I bought from a Kubota dealer that has a good reputation in that area and simply did a wire transfer. Not sure if it helps you at all, but figured I would at least share my experience