You can cancel a deal until the deal has been confirmed by both parties. This means that one party accepts an offer and the other confirms. After a deal has been confirmed by both parties you can no longer cancel the deal, and you have entered into a legally binding agreement to buy the item.
When you have bought the item you want, and accepted deals on similar items that you no longer want, you must cancel these deals before the seller confirms. If not, you have legally bought the item when the seller confirms.
You can cancel deals in the dialogue with the seller by using the “cancel” button if the deal is awaiting confirmation. You can also cancel by clicking the "X" on the product picture in your watchlist.