If you look at how KEH arrives at a "BGN" rating, you'll be amazed how really minor things are overweighted. Just make up a fictional lens and run it through their rating system and you will see. Their rating system severely overweights things that play no role in the actual value of the lens, in my opinion. And they do this in spite of a liberal return policy that should convince almost everybody that it's worthwhile to go for the bargains and simply return them if they aren't up to snuff.
The good news, for buyers, is that the lens you get, even if rated BGN, is typically very good. The bad news, for sellers, is that you get low offers for lenses that are almost perfect, and you'd typically be better off selling by other means.
Anyway, here are some things to look out for with RB lenses: check all the speeds (I used to use instant film to do this, and that has now become a bit too precious for that!). Also check the bulb and other settings. Also check that the lens mounts and dismounts smoothly... from what I've seen, sometimes people forget to cock the lens before removing it, then pry on it like a monkey and mess something up (albeit usually only a cosmetic issue).