The current fees for posting items on guild stores is much too high and will have a significant detrimental impact on the willingness of players to use the store.
15% posting fee which is non-refundable makes it very risky to post high level items. Even at low levels this presents an overwhelming barrier to entry.
10% cut of profit is reasonable, but combined with the posting fee this amounts to a 25% surcharge on a successful sale.
Since players are limited to only 30 slots in the store, a high posting fee is not needed to discourage junk posts. I would suggest lowering the posting fee and perhaps make this a sliding scale (the higher the asking price the lower percentage). Reasonable sliding scale would be from 5% to 2%.