You can sell almost anything on eBay -- almost, but not quite. You can't sell illegal items, like illicit drugs, or government-controlled products like liquor, tobacco, guns or prescription drugs. You can't sell lock-picking tools or surveillance equipment like wiretaps or hidden cameras. You can't sell human body parts or remains, like your extra kidney (that's been tried) or your dead uncle's foot, and you can't sell something that doesn't exist in some concrete form -- for instance, you can't sell your soul to the highest bidder. An eBay user named David Finn tried that and got shot down. Just for a glimpse into the complex nature of running a site like eBay, here's an excerpt from the letter Finn received from an eBay representative regarding the decision to pull the plug on his auction:
If the soul does not exist, eBay could not allow the auctioning of the soul because there would be nothing to sell. However if the soul does exist, then in accordance with eBay's policy on human parts and remains we would not allow the auctioning of human souls. The soul would be considered human remains, although it is not specifically stated on the policy page... Source: http://www.randi.org/jr/121203jref.html
For a complete list of no-no's, see What items may not be sold on eBay?. If your item is not listed anywhere in that section, it's probably okay to sell. The author has a refurbished Samsung Yepp MP3 player that she bought on eBay a couple of years ago and never uses. That's definitely OK to sell. In the next section, we'll list this MP3 player on eBay.