Who takes the risk of buyers not paying and sellers not delivering? What about the risk of prices changing in the middle of a flip? I'm afraid I know the answer already...
You sell the book first and get a quote. The quote is good for at least a week and in a decent number of cases for 30 days. As long as you can get the book shipped to you within that time period and ship it to the buying website, you should be good.
I understand the skepticism and that is why we have the free plan to test it out at low level.