Hi @Culcat and thanks for writing in with your question today.
TurboTax has a great article on taxes for Ebay sellers. You may want to take a look - it covers some important topics such as hobby vs. business as well as some common expenses that Ebay sellers might deduct. However, every business is different and without knowing more about your business it's hard to tell you exactly what expenses you might have. Broadly speaking, the IRS allows you to deduct expenses that are "ordinary and necessary" for your business. For an Ebay seller this would certainly include things like postage, packing material, Ebay fees, and merchandise that you purchase to resell. But it could include other things too, depending on what your business looks like. Is there a specific expense you're curious about? If so, please post a followup reply here.