Fun topic, I don't ever recall such details disclosed before.
I'll second pinniped : as airlines possess and exercise the right to alter the rules, we have an equal right to circumvent 'em.

I believe the story by 41*, but he got caught and thus is overly emotional. I'm yet to sell any miles, but I reserve this choice to myself only, not to "enforcers".

Let's learn some lessons:
-- private sale is preferable to broker or Ebay
-- sale within the same locality is preferable
-- lesser award amounts are less visible
-- don't risk more than you can afford to lose, i.e. - don't sell from your UA 1P account, sell 25K out of your 26K DL blue account

And, if worst comes worst - you don't have to came begging back to them, it's not the same as banned fr0m securities industry for life. Take your loss like a real trader - with a straight face. And if you really have to be a member - add/drop the middle initial, use a different address and simply create a new account. Hint - I have my first name wrong with one major airline, I tried for a few years to fix it, then gave up.