I have not tried them, but I have looked into it some and I would stay away, for one simple reason. If the sites have such money making potential, why sell them?
Repeat revenue is far better than one time revenue, and if you are worried about upkeep and maintenance, there are ways to do that cheaply. So if I owned a profitable site, I would hold it myself, not sell it.
My thinking is, if you want to make money with a product, or service, or unique blog/website there are cheaper and safer ways to do it if you do not mind a little work.
Setting up a hub page is easy, and so is setting up your own standalone blog.
Setting up a full e-commerce website is cheap too, $5-20 for the domain, and maybe $5-20/month for hosting.
Your primary concern will be SEO/site promotion and making sales or click throughs, which means having interesting content and relevant ads, or desirable products and services.