I am not the most extensive user of eBay, but I do use it successfully in certain areas.
I recently sold two very old agricultural tractors for what I thought was an exceptional price and would certainly sell another in the same way. I don't think I'd buy one as the bidding was so ferocious.
I also have an elderly Hewlett-Packard Color Laserjet 4500, which uses expensive consumables. I tend to buy them on eBay, as they are usually boxed and half price.
But be careful. Some items are more expensive than new on eBay, whereas others are very much cheaprer. There is no logic to it!