When D3 was being designed and tested there was one really major obstacle to figuring out how well geared players would be: The lack of a functional Auction House. As I understand it internal testers at Blizzard basically just played the single player game and based their tuning of inferno difficulty on their experiences in single player and this is a mostly useless data point when considering how things work with millions of players trading with one another relatively efficiently. I was talking with Ziggyny and Gnome awhile ago about this and we were speculating on how Blizzard could have programmed an AH simulator to try to understand what would happen to a player's gear when they had access to the AH. Ziggyny figured it could be done without much difficulty but I did not; I think you could get a decent first order approximation but rapidly the effects of efficient trade would be really difficult to guess because you would need a lot of data on gearing priorities, stat weights and other such things that are difficult to generate prior to launch.
Rather than try to simulate an entire economy though I think the best way to figure out the effects of efficient trade on gearing is to try to come up with some kind of multiplier to describe how much bigger a loot pool an individual has access to. If I play the game with four other players, one of each class, and we trade amongst each other with each person looking to maximize their own performance then my loot pool is certainly not five times as big as single player but it is much bigger than normal. If I am, for example, the Barbarian then any piece of loot with Strength but without any other primary stat will generally end up on my character. However, there are going to be times when loot with both Strength and Intelligence drops and an Intelligence using character puts it on instead of me.
Of course having an enormous pool of players helps in other ways. In the previous five person example when two sets of good Strength pants drop only one gets worn and the other gets discarded but in the real game both get worn. When levelling up I often find that I get a new drop and realize that it is literally five times as powerful as my old item simply because I have found no decent bracers while at the same time I discard my fifth good helm. I imagine these two scenarios to find some kind of equivalency:
Scenario 1: I use the AH and buy gear of value X for each of my 13 slots. 13 drops to fill up to a total value of 13X.
Scenario 2: I fill all 13 gear slots by getting gear corresponding to a random slot with random quality that varies. All slots start with a value of .5X and I stop adding gear when my total gear value hits 13X.
Loot quality varies from .5X to 1.5X: 21 drops.
Loot quality varies from .8X to 1.2X: 28 drops.
Loot quality is always 1X: 41 drops.
I think that the third scenario is very unlikely. Getting more random drops in a single slot definitely improves that slot slowly. .5X to 1.5X seems extreme the other way since it is highly unlikely I will find gear 50% better than my current gear and certainly not on a regular basis. I think the 28 drop number is pretty close so I will assume that being able to sell gear when I get multiple drops in a single gear slot is worth getting twice as much loot.
Figuring out the numerical value of having the right stats rather than just good stats is harder. Gear that has only 1 of Str/Dex/Int is only good for 1 or 2 classes but there is more than that. Some gear gives bonuses to specific skills, some gear is class specific and some builds use different stats than others. On one hand we have barbarian only belts that are almost never good because they are only good if they get barb friendly stats (high loot wasteage, little trade benefit) and on the other hand we have witch doctors who pretty much *must* use witch doctor only daggers (low loot wasteage, massive trade benefit) and as such the benefit of being able to get them from other people is immense. I think I can confidently say that even if an item is designated "good" I only have a 20% chance of being interested in it regardless though this number is clearly just pulled out of thin air.
Given one number that is mathematically sound and one that is made up I come to the conclusion that having an efficient AH available multiplies the loot available to a character by 10. That means of course that Blizzard would have to do some pretty hilarious testing if they wanted to see what it would be like for someone who farmed Inferno A1 for 10 hours and to get a sense of their gear. Here are 250 random rares - have fun! Having an AH is monstrously powerful for gearing up people quickly and I suspect that Blizzard really didn't anticipate just how powerful it would be.