There is always the solution of limiting the number of items of each type a person is able to have.
From what I have seen, Items come in "rare", "common", "raw", and "coin" classes.
If people were limited to say 4 raw items, 5 common, and infinite coin and rare items, it should help reduce hoarding.
That way, the more RP related items you don't have restrictions on, but items like raw materials that you AREN'T supposed to hoard, you won't be able to keep.
There are also usable items, but generally they have a restriction that either destroys them or returns them to a specific person, making them harder to hoard (hoarding makes them largely worthless)
Obviously, the values there are largely random and would have a lot of factors involved when deciding on them.
If the restrictions to items was placed on everyone, and then players like Udgard and Kragel had their values adjusted (or even all restrictions removed) to suit their role as crafters, since then would logically use the most raw items for RP, it would probably be best. Also, if the crafters had no restrictions for carrying raw items, you could reduce everyone elses max to 1 or 2, and they could just hand them over one at a time as they gather the ingredients.
This would effectively gut anyone who is hoarding and doesn't actually use items for RP (But it will probably cause some issues for you, Czez )
Since Hoarders won't willingly give up any items putting them back under the limit, it would probably be good to impose a time limit (say 3 months, to be fair even if they don't deserve it) after which ANYONE with more items than the limit allows for any category looses every item in every category, including coins.
IAB, you can't complain about this since you claim to be "Hoarding so others can't", this makes it so that no-one can hoard those sort of items, as well as leaving you with time to still make a profit. Guess you should be happy, since you are getting exactly what you claimed you were doing this for, to "Keep others from hoarding"