a) There are no other noticeable crown-sinks in the game and that was causing dramatic inflation. The Auction House was the next feature released and needed to sink enough crowns to balance out the crafting update.

b) It prevents people from reselling items that are only undercutting market value by 10% or less, enabling a highly unstable market that tends to vary wildly throughout the day and give advantages to certain time zones and huge advantages to people with no life who can be on for many hours a day.

c) It prevents people from making a 'career' of reselling auction house items (thereby both slightly raising and substantially stabilizing prices) by putting a dangerous gap between buy and sell prices for profits.

Also, you pay the listing fee -or- the auction fee, not both. Listing fees are refunded if the item sells, and you should price auction fees into your auctions.

I put it in a more than slightly snarky way, but really, I do believe that they considered both that it would discourage buying and reselling underpriced items as well as the crown sink aspect, though I could be wrong... my only evidence is that they were absolutely adamant about it being 10% even in the face of the feedback when the auction house was released. Perhaps they wanted a market that shifted around more, or just wanted to encourage players to earn money by playing the game not by playing material-tycoon.

However, it also means:
- People undercut by 10%, and 10%, and 10%, and nobody wants to buy to resell because they can't be sure if the market is really dropping or it's just a spike from some hasty sellers and the fees cut into margins too much to make it worth the risk ... while other people match price because that's what's there now ... so mats drop to half price for a few hours or a day until they get bought out back up to the normal stable price and stay there as long as people only undercut by 1 crown. Players end up either selling mats for less than stable price either because they are in a hurry or don't know they should wait for a recovery. Many mats really just don't -have- a stable price. (Is a swordstone worth 150 or 800? Depends on the day... it ought to have a stable price roughly in the 300-500 range given the spread of prices it covers, but it shifts around too much to say for sure.)

- People play material tycoon by spamming the town squares with offers to buy mats for stupidly cheap prices or sell them for stupidly high prices, presumably taking advantage of people that don't know how to change the sort type on the auction, or otherwise have trouble using the auction house. (And there are many such - a huge portion of the auction market behaves in a way that is highly irrational unless you imagine you can only see the first page of sort by end time.)

- Really good UVs and perhaps other really good items simply don't get put in the auction house to avoid the ten percent fee, they go to /2 or the forums, making the auction house just one of many places you have to check if you want a good item.

We do need crown sinks, but trying to put all the crown sink load on auction fees causes a lot of effects that I would consider negative. I was originally expecting a 2% fee just like the crystal energy market.

Of course, if the 10% fee is meant to stop market manipulation - which it would do if it needed to, or at least put a substantial damper on it; you need to buy out a -lot- to resell at your own chosen price - it's a little misguided because Brinks is there to make sure that the market cannot be forced high. Force one particular material high, and players will start cashing in their spark tokens on that material and the price will drop again. It's essentially impossible to have a material monopoly anyway. If they keep the 10% fee, they should take all Brinks' materials away, but of course, not without putting -something- in to sell for spark tokens instead.