I constantly buy / add to my wishlist older hardcover books I'm interested in reading and will come back and check when the used price is low and buy either Like New / Very Good / Good and have been satisfied with the results. Nothing like getting a 800+ page hardcover book with barely any use delivered to your door for 6-10$.
If one is worried about quality don't by anything used that is less than Very Good IMO.
Maybe the merchant is Amazon itself and hence a fairly reliable seller. Maybe the merchant is some other less reliable seller. Maybe the the merchant has the item in stock. Maybe it does not. Maybe the price shown is in the buyer's interest. Maybe it is not the same as someone else might see.
The difference for me, is that in general Ebay has structured its business so as to create little incentive for manipulating individual transactions in an extractive way. As a marketplace, Ebay tends to provide consistent and reliable information to buyers (as well as sellers) regarding price and the transaction history of the counter party.
Now all this is with the caveat that I generally use Ebay as a replacement for Amazon and therefore am not heavily invested in the auction process. Usually the only time a enter an Ebay auction over buy-it-now is when the auction is ending soon and there's not much bid activity and the item is on the long tail...I don't use usually use Ebay to buy high demand or high ticket items via auction.
But again, for me it is that I find Amazon's purchase process is often untrustworthy. Essentially, Amazon's search is broken when measured against my economic interests (and great when measured against Amazon's). In short, Amazon has become an adversarial marketplace where my adversary is the house. Ebay's auctions are adversarial, but I'm not up against the platform and the economics of the transaction for me do not depend on how the platform leaks information...and there are few incentives for Ebay to evolve in that way and many incentives for Amazon to do so.
For many years Amazon's business model was to generate revenue off of straight forward sales transactions. Today it has shifted to rents and extractions. Prime is perhaps a good first example. Using it complicates transactions and Amazon can arbitrage the eligibility and price of items to extract additional revenues on the fly.
However, textbooks are the prime of example of Amazon's new more extractive business practices. It's not just that Amazon wholeheartedly adopted the text book industry's obscene pricing models rather than disrupting them. Nor is it just that Amazon has extended the obscene pricing to rent out ebook versions of textbooks. No, the best expression of Amazon's newly found extractive business practices is in the availability and pricing of used older editions of textbooks on its platform. In the old days, Amazon was a steady source for fairly inexpensive obsolescent editions of current textbooks. Once Amazon seriously entered the textbook business, that seems to have changed and bargains available with deliberate search have disappeared.
There really was a change of mindset with Amazon Prime. It was a manifestation of a business cultural value that all customers are not equal as individuals. Maybe it is possible to position an ecommerce site as an upscale brand while still selling Schenzhen's spinners and iPhone cases and unlisted electronics of dubious safety for $0.01.
As I've lost trust in Amazon, I've started using Ebay for online purchases. Ebay is orders of magnitude more transparent. Transparency is a good way to create trust. It helps that Ebay's business model is not grossly misaligned with my wallet. It helps that the marker of trust is between buyer and seller rather than stars based on customer reviews. A seller with 10,000 transactions and 99% positive rating is a better indicator of an item's quality than 4 stars based on twenty reviews.