The 13% is what Gemm will take from the seller.
So if you sell an item currently advertised for £100 they will take £13.
If the seller wants to increase the item by 13%, from the date the new policy comes into play, then let them know.
So a £100 item for sale now will become £113.
Gemm will still take 13% of the £113 if you sell it (or £14.59 on this case)
Thats my understanding anyway.
Hope that helps.