The short answer is anything. You can make anything and sell anything on ebay. With that said, there are rules and policy you must comply when listing your items. But, again anything, well obviously some items such as a liver, a hear, human bodies parts, you can list and sell but others can complaint and your listing will get terminated or account banned. But in term of making something to sell on ebay it’s almost anything being creative like this armored armor for your guinea pig LOL

guinea pig ready for battle! LOL armor for sale on ebay