Well, it looks like you've only had them posted for a little over a week. I have seen wheels stay up for sale here at S197 for months before they sold. It took me forever to sell a set a couple of years ago, and I ended up selling them locally at a yard sale of all places!
I'd throw them on CL, and maybe other Mustang forums as well. As far as the price goes, IMHO, $650 may be a tad bit high, considereing you can get brand new wheels from American Muscle for that price, and even lower, depending on the style. But again, that's just my