I havent ever sold any of my Miles or Pts and have no plans to do so.
However if I was to, I would only do it if the broker purchased ALL of the miles in my acct, that way if the acct is Closed or Frozen they lose and not me. A check must be paid up front for all my miles or pts and I would Never do it with any acct where I still planned to fly or earn status.
So if I had say 100k in UA and dont have any status with UA and dont plan to fly them then I would sell the Miles. If Im a Plat and plan to continue flying UA then Id Never sell the miles least I be shut down, for sure if I had 450k I wouldnt sell only 100k least it hit the fan and I lose the other 350k