AuraSeer said:
The other side of that coin is that it'll be hard to find one when you need it.
Right, which is why saying "-10%" is a bad thing. In that sort of situation (needing an item that's restricted) the price should be HIGHER than listed, if no non-restricted version of the same item exists. (Hint: no non-restricted versions of the same item should ever exist.)
My point was, use restrictions shouldn't be considered at all when determining the "value" of an item (value != price), or how much XP it costs to make, or how much raw material. It should only be taken into account when comparing supply vs. demand.
If the player wants to unload a restricted item onto a shopkeeper that doesn't need it, or if the shopkeeper is trying to sell a restricted item he bought, the price should go down.
If the player needs to buy a restricted item or the shopkeeper has found a rare and powerful item that happens to have a restriction, the price can go up.
Now, you say, "but almost all of those methods end up hurting the player, why would I make an item with an alignment or skill restriction?" Answer: YOU DON'T. Your DM designs all items (cue dcollins!), and will decide if a restriction is necessary based on the concept of the item (Scry for a crystal ball, Good for a Sunblade).