Here is one you may not have heard of - vintage manuals. These can be repair / maintenance manuals, instruction manuals, or just plain owner's manuals. These can be collector's items. The best selling niches are automobile manuals or vintage electronics manuals. Here are a few completed listings:
1978 Apple Computer Manual - sold for $49.99
Austin Mini Cooper Repair Manual - sold for $41
Look for these items at thrift stores, garage sales, estate sales, library book sales, and used book stores. These manuals are an excellent example of an item that looks like trash - something obsolete - but has value to collectors. Learn more about "throw away" items you can sell on eBay by visiting my Facebook group of over 2,000 sellers.
For more niche items to sell on eBay, check this resource: