I noticed that green and blue items are not bought even if they have a low price at auction.
I have always put them up for auction believing that they were useful for those who have enchanting or to others who need to change some piece of equipment.
But it's clear that I was wrong.
I've noticed that you make more gold and quickly by selling everything to an npc and considering the time not lost in opening each auctions, especially if you have a lot of items and then also considering the savings from gold not spent on each auction.
Mostly you sell a few purple quality items, but even those are best sold directly to the npc, except for specific items that are required.