I would think that ratio is much higher on SR since the vendors rep are what customers look to when deciding what vendor to purchase from.
So the vendors want to get as many good feedbacks as possible, they often ask the customer to give feedback before the item has been delivered(!).
Still I dont think 100% of the customers leave feedback.
For some reason I was under the impression that SR requires you to leave feedback before releasing the funds from escrow.
I do agree that there's no way to gauge the amount of private sales activity, so figures based on public sales will always be on the low side. That true of Bitcoin itself, too, though. Not all trades go through exchanges and private trades can't really be tracked.