It really all depends on what you are seling. Everything has a value. It just depends if that value is worth it for you to go through the effort of listing and shipping the item. I have sold several textbooks and rare toys online.
It's all about supply and demand. Toys, consumer electronics, and textbooks tend to sell well across most websites. If you have something that's vintage or collectible, then chances are high that you'll be able to sell such items with ease at a high price. You're not going to find much for common or household items, though.
