Sounds like a good deal to me. Considering that most Knives that you see for sale are CKs, ABK, and BGKs...it's not common. The other thing is...a lot of South American Knives have a "Zebra" pattern so the common name may not be the species you have. A picture would help, but what I was able to find under Zebra Knife is:
Scientific Name: Gymnotus pedanopterus
Max Size: 12"
Minimum Tank Size: 50g
Temperature: 72F-82F
Feeding: Offer it live and frozen foods such as bloodworm, prawns, earthworms, tubifex etc. Dried foods are not usually accepted.
Tank Set Up: Provide plenty of hiding spots. Noctural in nature, so dim lit tanks would be best.
Compatibility: Will eat fish small enough to fit in it's mouth. Will get a long with most fish the same size and larger, however, very boisterous and aggressive fish should be avoided. Keep with larger Characins, Cyprinids, catfish, Loricariids and Polypterus. Like most South American Knives, this species is very territorial with conspecifics and one Knife per tank is highly recommended.