I had that recently happen to me. A friend asked if he could buy some of my miles. I just sort of shrugged off the comment, but was later annoyed by the request and started thinking...

I generally redeem for international trips giving me 6-7% return per mile, however I recently redeemed 150K BA miles (with a companion cert) for two First class tickets worth $24K. I will get 30K redeposited back into my account due to a Chase promo, so my net is 120,000 BA miles. After I deduct the fuel and tax charges, I'm getting $22,808 in airfare for 120K miles. If someone wants to buy my miles, my new price is 19 cents per mile.