What you mention happens occassionally on America's garage sale, eBay; even though usually prices on eBay end up higher than classifieds. The main reason this happens is bad spelling so folks who search don't find it, or it is in the wrong catagory.
I picked up some $400 audio cables for $65 because they were listed under Car Stereos. The seller did not notice or even seem to care they sold so cheap? The classic Rotel RCD-855 I use at work was listed at Consumer Electronics Other and was spelled Rotell. I paid less than half of what they sell for used usually. It was mint, and had the manual, remote, transit screws, etc. The Conrad Johnson preamp (with phono) I use in my small home office system had a "Buy it Now" price of only $350. New list is $2250. Last was a Yamaha TX950 tuner I got for $175. Did not care for it and resold it for $350.
To find this stuff you have to search for what you want at the main home page, so it searches the whole eBay database. Good Luck!!!