When you're listing an item, you will be asked to set a price for it. The price you set is entirely up to you, and the amount your item sells for will be donated to charity.
While you may wish to donate a large amount to your chosen charity, the price of the item still needs to appeal to the buyer. If you are not sure about how to price your item, research what others are selling a similar item for. As a rough guide, it is usually about 50 percent lower than the original retail price.