"Only 5%" and then another 5% is a large chunk of expensive items. I would argue that it does the opposite and encourages disproportionate pricing and jamming up the market. It does nothing to encourage people to list at reasonable amounts, because the people that sell high will sell high anyway, and it punishes the people who actually are keeping the market flowing.
10% is really huge. Other MMOs have fees in the 1% range.
But I don't even care about the fee so much, I don't want to be punished for re-listing an item.