Most professional graphic artists (people who actually do graphic design as a living such as logo design or landing pages and the like) usually require payment first and then offer "x" amount of revisions or sometimes even unlimited revisions. They also usually allow unlimited minor revisions such as colour changes or slight size changes or something of the sort.
It would probably just be best if the graphic designers here just do the same thing. If they already have an established portfolio of good work, then there's no reason for me to not go first to them. If I have a general concept already and then have them create a basic mock-up, I should be allowed to give more specifics as to what I want and then they should be able to revise afterwards to match my descriptions. I mean like most of the time people don't end up liking their final design because they aren't specific enough as to what they want.
"Oh, yeah, I just want like a flying monkey or something like that dancing on a book"
"Sure, here it is"
"I can't accept this, this isn't what I wanted the monkey to look like. This isn't how I pictured it. The book's not even the right colour!"
It's kind of hard for an artist to design something when they're expected to read the person's mind. If the graphic designers just offered "x" amount of revisions, then the buyer wouldn't really have a choice other than to pay and be as specific as possible or else it's just really his problem that the final design didn't come out the way he wanted.
Just my personal opinion is all, I buy a lot of logos/banners/custom graphics from a lot of people and this is usually the way it's done. It could already be implemented here on Sythe for all I know, I don't really check the graphics marketplace. It would probably save both the artist and the buyer a lot of time with the complaining and payment issues and whatnot though.