Well, 15% isn't a fixed number, so it has no relevance to the sellers posting value. If it sells for $2, 15% is .26 cents. If it sells for $30, 15% is $4.
A listing fee or deposit is what would effect the minimum price for the seller. This however would discourage selling as real money is involved. I certainly wouldn't post a digital item from D3 if I had to pay to do so, as the item may not sell. This is the WoW AH model, but does not deal with real money.
Unless there is something I haven't seen, the value of items on the D3 RMAH will be entirely player driven. The people that do buy items from D3 RMAH will dictate the value of those items simply by paying what they are willing to pay for them. If an item doesn't start selling until it's posted at .25 cents, then that's the value.
I'm sure some twit will be willing to pay $10,000 for some uber item. Does anyone really care or feel slighted because they didn't get the item, even though you probably never would have anyways? I never saw every item in D2, especially since many weren't even in the game until after I stopped playing. So why should I care if some super uber item is found in D3 like once a month across the entire game and then sold to the willing for masses of money? I just hope that I'm one of the lucky to find whatever that item is.
Hell you never know, crafting components could become a high quantity mover and everyone could make a little here and there if they wanted.
Biggest mistake people are making though is comparing the economics of the WoW AH to the D3 RMAH. Fake Money vs. Real Money. Behaviours in D3 from both buyers and sellers will be far different from those in WoW.
However, there is still the Gold AH in D3, which is more comparable to WoW. How having two very different AH's affects one another, that will be interesting.