I, myself, have bought a lot of import games off of eBay, and have never gotten a bootleg one. My best advice is to compare different photos from different auctions, if you're going the eBay route. Legit games and cases will usually look much shaper / cleaner than bootlegs, and are much more common. Also, make sure to buy from someone with a high feedback rating - hit-japan is a longtime, trusted seller from Japan.
If I remember right, Saturn games are hard to bootleg. Each disc has a sort of "stamp" on it at the outer edge, that more or less tells the Saturn that it's a legit disc. It seems like a little thing, but I don't know if it's even possible to duplicate that.