I've used the tracking number to trace a parcel that was sent to me (not via eBay). When it clearly had gone missing, though, I went back to the seller for a replacement, and let them sort it out with Parcelforce, since as you say they were the ones with the contract.
When Parcelforce let me know a month later that the package had mysteriously turned up, I told the seller so he could get it back. He was surprised I hadn't just taken it myself; but I figured it was his, not mine.