1-Ebay publish their fees and they state they take % of shipping as well

2-It is your responsibility to price your item so that you receive what you want after all the costs

3- The simplest rule is to add 20-25% to the amount you want to end up with for your item to cover those fees

4-Only use eBay calculated shipping if you are using eBay Postage

5-You are not going to find another platform that facilitates selling used books worldwide with the number of buyers eBay has