I have an aquaintance who does that sort of thing for persons who deal in high value cars, etc. But I have never done business with him from the selling end. (In other words, I've connected him to some owners of rare cars in the past.) Beyond the percentage fee, there will be a contract and fixed fees for a variety of his services.
You might want to look at Show Your Auto or a similar site to see how they handle such things. It's been a while since I read through it, but Barret Jackson's auditors have posted a copy of BJ's agreement as a part of their report in the past.
As John seems to imply, depends on what you are selling... and how many... as to whether that would be a good route for you.