I've not bought with a "D" rating but have bought two lenses in the last few months, one an enlarging lens and the other for the RB67. They both had the "E" rating and were extremely clean. The glass is perfect on both. I used Paypal and the first time it took a couple of weeks to go through, I had never had that happen before. Every time I checked it was in "Pending" state. The second time it went right through, maybe it's fraud protection. I am completely satisfied with both purchases.
The KEH purchases were not so nice, a tripod head had a bent handle and was roughed up and a lens was very well used by the Miami Herald, as inscribed. You know that a newspaper would used the equipment intensely. In all fairness though if you don't go Excellent or better you just don't know what it will be like. When they say RB back it doesn't mean it will have a dark slide, which most would assume it would have.
So Adorama gets my vote.