Flippa charges $49+ for listing, depending on what features you get. They keep a percentage of the sale upon selling too, yes. The site transfer process is about sending the site info/details to the sellers. There are a few ways to do it. It takes me 5 minutes to do, literally. Really simple. I have hosting with Siteground. Most of the sites I sell have content but, I have sold some with none. I'd recommend adding content.