Ducats or Plat tells you which items should be sold for ducats and which should be sold for platinum based on the current market value. The plat prices come from Warframe Market or Nexus-Stats.
Lootr tells you the best mission to farm any mission reward in the game. Enter an item like "Axi V7 Relic" and Lootr will look through the drop tables of all the missions to find the one with the highest drop chance for that item. The drop tables comes from the Document published by Digital Extremes