I often try to sell "powerful" rare items on the auction house. I generally look for items that have high numbers for primary stats. For instance, if I'm playing a dexterity-focused character, and I find something with no dexterity, but +50 intelligence, I might opt to sell that on the auction house.
However, I really have no idea what the valid range for these enchantments is. I'd like to figure out when I'm actually holding something that is above average, versus just sort of guessing. It strikes me that the more "awesome" a rare item's enchantments are, the higher I should try to price it on the auction house. It would also be useful for sorting the "resaleable" rares from the vendor trash items.
I know, for instance, that there are minimum item levels for certain enchantments altogether (ie, nothing below 15 has sockets, and +resist gear doesn't show up until Nightmare levels). However, as item levels increase, does the valid range for the enchantment also increase? What's the formula?