I’ll submit that you’re experiencing the “Honeymoon” effect.
When a seller first goes and sells off their private book collection, there are a bunch of high-interest titles in their collection. If those titles are expensive enough to make it worthwhile for customers to buy them on Ebay (no one wants to buy a $1 title on Ebay if they can get it elsewhere), they sell like hotcakes.
This makes Ebay look like a good venue to sell books.
There are a couple of problems.
First, though you might have gotten good money for this book on Ebay, most of the time it would have gone for even more if you’d listed it on a fixed-price site like Half or Amazon and just left it up for a month or two.
Second, in two months all your hot books will be sold and you’ll be left with either worthless slow-movers or expensive slow-movers. Most booksellers have at least some inventory that would only sell 6 times a year, at any price, and most of the time the buyers never even go to Ebay.
Still, if it’s August 16 and you’re trying to unload a Biology textbook that’s not yet obsolete, you may get top dollar.