Amazon is an online retailer. Because they are so huge, they can supply the infrastructure for smaller companies and individuals to sell on line. A bit like e-bay but without the auction.
So, as I understand it, Amazon Marketplace is somewhere for individuals/small traders to sell things on line. Presumably Amazon takes a cut. I expect the contract will be with the seller not Amazon
"It's cheaper to do it on eBay."
You would obviously know boxtops.
Presumably you also sell on Amazon, as some people are reluctant to purchase on eBay. (I am, for one, especially as PayPal can't get their heads around the concept of someone living in Greece and banking in the UK).
I buy many books from sellers in Amazon marketplace. It is cheaper than eBay. I look for books at 1p each and £2.80 p and p. Don't think you can get any cheaper than that - £2.81. Charity shops might but they have not got the wide scope which Amazon marketplace has. So far I have found them more reliable than eBay sellers.
I thought that boxtops then I realised that second class was fine so all of my books (some of which are very heavy) have all gone within the delivery charge; give or take 50p.