The reason why this can't be done strictly depends on the type of item we are talking about.
Fansite items are the one that you can not sell via Market. I remember why they were removed from it. It was few years ago (2015 or 2016, I'm not sure) and there were drama that all contests are frauded, that admins let win their friends best rewards to make cash in game. You know, that time fansite items were more prestigious and it's price compared to other items were impressive - nowadays eq is more expensive xD). Because of this drama CMs decided to remove it from Market to make selling fansite items harder.
Another type are items where you can write something - goblets, Miniature House, contest's rewards like medals or trophies, tapestries of honour and more... It's because these type of items are usually highly personalized, so they are not intended for sale.