I have been listed used books on Amazon however there is one company which often appears. They are called NearFine. Their Amazon profile shows they have 32,629 reviews. The average is 1 review for every 20 sales made so the average would be 652,580 sales. Their website claims they have sold 894,637 books so those two figures appear to accurate. https://www.nearfine.com
When you look through the books they sell they are highly valued. From $20 to $500. However when you click on the books they are selling their prices are much higher than the cheapest price.
This book for example is being sold for $16 however NearFine have it listed $175.00 and yet the number of reviews they have shows they are making sales.
Podracer! Coloring Book Star Wars: Episode I...
If you took an average of $30 profit they would have made $268,000,000 based on the number of sales they have made.
How can they sell books ten times the value of other sellers?