Bad DVD

A couple of months ago, I bought the DVD set of Season 4 of Buffy the Vampire Slayer. I bought it (brand new, still in plastic wrapping) from an ebay seller. Today I discovered that one of the disks has some problems. It skips around, goes fuzzy, etc. Maybe it's scratched? I don't really know anything about DVDs. What I want to know is, is there anything to be done about this? Can I call the manufacturer or something for a replacement disk? It was definitely defective when I got it -- yesterday was the first time I ever touched the thing. Anyone have useful advice?