Point 3 is interesting. I had always sensed legendaries were more frequent when I have collected excess gold and/or gems. Alternatively, when my gems and gold totals are exhausted I seem to get offered less legendary items. I had never related legendary availability to buying and selling.
Point 2 is an interesting insight. I had not noticed the sale value was set at 10%.
Additionally, I understand that Epics can be sold far more frequently by players that spend over and above VIP. Those of us who do not buy packages, heroes, etc, do not get the same opportunity to sell epics. Instead I have a virtual shed storing my 500 surplus thighbones.