I've been buying cameras on eBay for about twenty years -- I've gotten a couple duds, but in almost every case was able to either return it or get a refund (and most of that time I've paid only lunch money anyway). I have not bought film on eBay, but I wouldn't hesitate to do so from the accounts of retailers like B&H, Adorama, or Freestyle (where they might sell items they don't want to offer through their store or web site, due to expiration or uncertain condition, but note that in the eBay listings). I'd be very cautious buying film from a random seller on eBay; at the very least, I'd ensure that it's not a seller in China or other Far Eastern country, where one might get a 35 mm cassette containing multiple very short film strips taped together (I've read about this here on Photrio), or film that's completely not what the seller said it was.