Thanks for your super quick responses!
(1) Looks like the "norm" (if there is one) is about US$0.01 to US$0.02 per mile. So if I go ahead with this I'll probably just price in the middle at US$0.015/mile.
(2) I am curious as to why selling miles like this is not a good idea, and how it is worse "especially" since I posted the itinerary. Can you elaborate on that? I just really don't want to end up doing the opposite of helping a friend. Thanks!!