I mean it's not an upper hand kind of thing. With amazon, once you ship and the seller gets the item, you get paid minus amazons commission. But amazon does reimburse you a bit for shipping. As long as you get tracking for all your shipping, you should be fine. Whereas ebay, you have to pay ebay commission and paypal commission. I don't ever think paypal or ebay protects sellers. Had a bunch of bullshit with them. Also, if you want to save money, get online postage. Amazon lets you get it straight from there.
Yeah in amazon, they basically calculate everything for you, shows you what you will get after item sold, minusing the amazon fees. You need to prove a track code to prove you have sent item to buyer, so must send as 1st/2nd class recorded delivery. I don't know about business sellers though. I sell in amazon, they take 20% away from you, then you need to pay postage for book as well. NOrmally amazon's fixed postage would be about £2 something, you dont get a lot from it. The tip is to sell expensive books only, if you are not going to earn anything, you might as well donate the book.
