Nether Stars, Beacons & Elytra (unenchanted) can be reverse auctioned with a minimum quantity of 5.
Enchanted Items can be reverse auctioned in any quantity. You may mix multiple types of enchants/tools. You must specify the acceptable range of every item's enchantments (i.e. a Silk Touch pickaxe with either Efficiency 4 or 5 and Unbreaking 3) and they all must have full durability.
Potions can be reverse auctioned in minimum quantities of 54. You cannot mix potion types in a single set of 54. Zombie Virus' are considered potions.
Horse eggs can be reverse auctioned with a minimum quantity of 10. You must specify the acceptable ranges for each of the horse's stats, including speed, health, jump, and type.
Horse armour can be reverse auctioned in minimum quantities of 10. You can mix the type of horse armor (eg: diamond with gold), but you must list how much of each type you are reverse auctioning.