If the problem is on the uncoated glass sides facing the focusing screen or the eyepiece, then it may be possible to clean these surfaces. If there are any problems with the coated sides of the prism, they are likely due to de-silvering and there's not much you can do there. This is probably the most common problem with these older prisms, as the foam pads holding the prism in place deteriorate and damage the reflective coating, or the coating begins to separate along the corners of the prism. If it's on the glass elements of the eyepiece, these may be possible to clean. These aren't necessarily easy to open and service - you want to be extremely careful to avoid further damage to the prism.
If you can't inspect the prism personally to assess the damage, I wouldn't buy it as the problem may not be repairable. I have several F plain prism finders with various issues which I've obtained cheaply over the years, only to end up paying for clean undamaged ones when I found them. I'd wait for a cleaner example and pay a bit more money. I'd rather use a bulky metered finder with a dead meter and a clear view, than a prism finder with coating damage or fungus.