I too have used Fiverr back in the day when I got started, sadly I have to agree on most points you have made. However, I am always extremely cautious to judge or dismiss something as ineffective. There’s always the chance that it didn’t work for the both of us and thousands of other people. But there’s always the chance that it did work, and perhaps still does for thousands more.
Hunt for under-priced used books that you can sell online. Download an app that reads ISBN numbers so you can scan the barcodes on books. This will pull up the book’s current price on Amazon so you can see if it’s worth trying to resell it. Then, visit used book stores, thrift stores, and garage sales to look for high value books. Post the books for sale online using sites like Amazon or Ebay.