Note: This is mostly for active items more than items with an Aesthetic Effect.

Since Lioden is content giving vague and half-stories, I've taken it upon myself explain exactly what they do. Some of these items I do not have, but have used enough to know what they do. Others I have vague memories of and would not mind some assistance. Right, let's begin.

Argan Oil make's the old age on elderly lions disappear. It does NOT de-age them. There are no de-aging items on Lioden.
Violet, Lavender, and Indigo Eyes Changes a lion's eye color to whichever one is choosen. Lions cannot pass this color, only fail colors.
Poses Poses for your adult female lions. They inhibit mutations and cannot be used on mutated lionesses. Also, they cannot be removed on a whim. Removing poses costs 200sb.
Marking Remover Removes one mark from any lion.
Random Eye Changer Changes lion eye color (in my experience, never a special eye color). Can be bought for cheaper at Monkey Business.
Random Marking Applicators Chooses from Custom and Random marks at random opacities. Almost never chooses any other kind of mark.
Scar Bundles give you three random scars. Scars can also be gained in: Sub male sparring, lioness encounters, NPC battles, bought from the TC usually for way less. Don't waste your GB.
Lab Test Frog Lion will show as the opposite gender without actually being so.
Marking Opacity Changer changes the visibility of a mark. Can be bought for cheaper at Monkey Business.
Eyes Changer changes a lion's eye color, but includes several special options sans Sunglow.
Mane Color Changer Changes lion mane color-- all custom colors.
Base Changer Let's you change your lions base color -- ONLY custom colors.
Mane Changer Only Custom mane shapes.
Tigers Eye keeps a lioden in their adolescent state physically. Does NOT stop aging.

CRBs do not stack.
Vuka Vuka does not include Goddess in it's fertility choosing options.
Oasis Jellyfish claims lions are not trade-able after being frozen; they are. Also, only works on adult lions. Cubs can only be frozen with the normal Jellyfish winnable from Slots.
Auto-nesting will not work if your lionesses are already on their last day.
Feed All does not feed the entire pride at once; you must choose which carcass you will feed your pride from your horde one by one.

Feel free to correct anything I've posted in your replies!