You obviously didn't bother reading the link. Both parties add collateral ideally equal to the item price: so the buyer pays 2x the price and the seller pays the list price and sends the item. If the buyer is happy they've received the right item without issue both click a button to release their collateral back to themselves and the buyer's money to the seller. If not they have to agree how much to release to each other since they both have skin in the game.
So in your example you can sell an ice cream for £10. When I buy it for that I pay £20 and you pay £10 and send the item. I eat it and we're both £20 down. So I want my £10 back and click a button. It then releases £20 back to you - the item price and your collateral, and my £10 collateral back to me.
So in your example you can sell an ice cream for £10. When I buy it for that I pay £20 and you pay £10 and send the item. I eat it and we're both £20 down. So I want my £10 back and click a button. It then releases £20 back to you - the item price and your collateral, and my £10 collateral back to me.