There should be a warning message if selling an item for an amount less than its estimated value. Since estimated value is already calculated, the warning could be configured as no less than 90% of that, 75%, 50%...
Context: I just lost a reaper mask worth 21m because I was selling a bunch of beheaded horrors. Since both textures look identical, I listed my reaper mask for 110k (the value I was selling beheaded horrors at) and it got insta-sniped by a flipper :/