Second Hand PC & Warrenty Information - What Do I Do?

Hi. I have recently purchased a secondhand PC from a seller on eBay. And, there are major things wrong with it. But, of course, these faults were already known to me at the time of purchase.

I have access to Service Tag and Express Service Code information. It is present on my screen. I haven't gone any further than the on-screen information you see. Do I still need to contact the eBay seller for further information, or can I proceed on from there? This is my first time at this.