It's way easier to sell a problem-free lens. By the time you describe the issue, then protect yourself from the idiot who buys it without reading, I'd just fix it myself. My guess is, $30 since you don't exactly know what's wrong.
To fix yourself, it's probably worth buying some JIS screwdrivers
like this, though all you actually need is the smallest one of the set. I have one of these and would be happy to take it apart more than this. Because it's a macro and extends a lot more than an ordinary lens, it's not quite the same as other M series. The first photo shows the mount and aperture ring removed (5 screws, take the mount off, then carefully slide the aperture ring off and look for the ball bearing):
Note that a lot of M series lenses have the ball bearing right under the orange diamond, but here it's offset to the side where the ivory locator bump thing is. The bearing on your lens should stick out like this. There's a flat spring-steel spring behind it, normally shoving the bearing out into teeth on the inside of the aperture ring. You can see a rivet for one end of the spring to the left of the bearing. If the bearing gets stuck in old grease or dirt, it won't be pushed out enough and the aperture ring will have a sloppy feel to it. Anyway, it's easy enough to check if you can get the mount screws off.
Here's the front just for completeness. As mentioned above, the whole "cone" unscrews. It has its own threads cut just inside the filter threads. Because it's got a large surface to grip and the threads are less likely to be dirty or bent, it should come off easy.
Inside, the lens is a bit weird, because of the macro thing. Another assembly quirk is some screws hidden under the rubber focus ring grio. I forget exactly what they do.