This is a valid point. The reason is that there is a manual process of getting the book shipped and then shipping it. This cannot scale very well. I can may be trade 50 books a day but that is the upper limit. I don't want to invest too much time into something that can't scale. By opening it up to the world with a monthly membership, I can take a vacation and still make money from this.
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...
Since you are buying and selling from/to businesses instead of people, there is a low chance of things going wrong.
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.
Does that sound good enough?