Astronautic Engineering - 20 bil
Mobile Refinery Operations (even more rare than AE, removed back in 2004) - 35 bil
M̶o̶b̶i̶l̶e̶ ̶F̶a̶c̶t̶o̶r̶y̶ ̶O̶p̶e̶r̶a̶t̶i̶o̶n̶s̶ ̶(̶t̶h̶e̶ ̶r̶a̶r̶e̶s̶t̶ ̶o̶f̶ ̶t̶h̶e̶m̶ ̶a̶l̶l̶,̶ ̶o̶n̶l̶y̶ ̶f̶e̶w̶ ̶e̶x̶i̶s̶t̶)̶ ̶-̶ ̶5̶0̶ ̶b̶i̶l̶ (SOLD)
Those skillbooks were long ago removed from the game. MRO and MFO can’t be sold through market. Skills don’t do anything except hanging in your skilltree and making that extra skill in your total that makes you special and separate from the crowd.