Interesting topic, I find the percentage rating system as problematic. The system is totally subjective and based upon the perception of the grader. Another problem is the nature of the wear. For example a gun may be rated at 95%, if the 5% of wear is normal holster wear the gun probably doesn't look too bad. If the wear is a pitted rust spot or a flake of nickel off the side plate the gun may look awful. If the % system is used I think there are a few useful levels, I would only use 98%, 95%, and 90%, below that the gun is a shooter so % is less useful. Be especially wary of guns described as 92% 96% etc., such minor variation is about impossible to judge.
Rather than the subjective % system if the gun is not available to inspect I would prefer good clear photos and a detailed description of any wear, damage or problems, this is a more objective method.
I buy and sell many used guns for a major retailer and have examined thousands of guns in the past 5 years and still don't feel comfortable with the % system. If a customer calls about a gun I much prefer to give a detailed description of the gun and send good photos if necessary. If I am pushed to give a rating I normally offer a very conservative figure, I would rather have a pleasantly surprised customer than a disappointed one.