How do I filter items in the Auction House by additional damage?
Solution 1:
A change for this was introduced with patch 1.02 today.
The "Physical Damage" Preferred Stats option for rings has been renamed to "Bonus Minimum Physical Damage"
The "All Damage" Preferred Stats option for weapons and armor has been removed and replaced with bonuses of specific damage types
You should now be able to search for Bonus Minimum Physical Damage to find those +Damage Rings, Amulets, offhands, etc.
Solution 2:
In one of the greatest oversights possibly imagined, there's currently absolutely no way to search for or sort by the damage granted by off-hands, only the other stats you may find on them. I'd suggest posting this on the bug report forums in the hopes that in a future patch they'll add it in - not having it makes acquiring new offhands a rather irritating affair.
Solution 3:
As of the 1.05 patch, it is possible to search off-hand items and jewelry with an attribute of "Average Damage". This is the middle of the + damage range for the item. So an item with +10-20 damage would have Average Damage of 15.
Solution 4:
There isn't a way to do that. "All damage" as stated about is not weapon damage. It may be reduction or something, but definitely not damage.